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Why birds can fly over Mount Everest (2020) (nautil.us)
250 points by Phithagoras 56 days ago | hide | past | favorite | 108 comments

an interesting thing is that though birds have super-efficient lungs, they can be very fragile.

As any parrot owner knows, if you overheat a teflon pan, all the birds inside your house will die. So bird owners use stainless steel. There are other non-stick pans available, but it's really hard to wade through the marketing to figure out if overheating will kill the pet that's been in your family for years or even decades.

unrelated, the name of this article made me wonder if it was about thin air and lift, thinking how helicopters can't fly over everest.

But a mountain like everest might have a huge amount of updrafts to lift birds into the sky.

If you've ever stopped at an ridge overlook along california highway 1, you might see something curious. the ridges are bird-superhighways. the ocean air hits the cliffs and deflects upwards, and this updraft lets birds "surf" up and down the coast. So if you stand there and are patient, you'll see lines of gulls cruise by going north or south without flapping.

I think the wrong assumption here is that we humans are completely fine with teflon overheating on the other hand. PFAS chemicals released really do mess with just about everything and anything in our body, the results just aren't seen on short time scales.

We really should be taking steps towards a worldwide ban of this yet another asbestos-ass miracle material which sounds too good to be true because well, it is.

Some ovens are teflon coated, and some of those have cleaning modes that effectively kill of pet birds.

I don't get how that can even be allowed ad a regular household appliance.

Alcohol kills humans in relatively small quantities as well, and it would certainly kill a bird with significantly less. Chocolate makes dogs incredibly sick as well, while not having the same effect on humans.

This isn't to say that we shouldn't ban PFAS. They're proven to be dangerous to humans, and we should absolutely be concerned about long-term effects. It's just that "it kills a bird" isn't a justification for banning something. I'm under the impression that burned food and smoke isn't great for you either; it's possible that the trade-off between a dirty oven and increased PFAS exposure is acceptable.

Ultimately it depends on the magnitude of the danger, and I don't know enough about airborne PFAS to know if it warrants a blanket ban on any products with Teflon.

> It's just that "it kills a bird" isn't a justification for banning something.

I don't know, it killing lots of things with lungs seems like a red flag to me.

Perhaps it warrants switching the assumption "Teflon good (until proved harmful)" to "Teflon bad (until proven safe)".

I'd be open to that viewpoint. Assuming something to be bad based on an animal trial is generally how we approve medicines, so there's precedent for it.

I will say that we should be wary of banning products based on potential long-term effects with unknown magnitude. Knowing where to draw that line is tough.

It's already proven not to be safe. If you overheat a pan and breath it in you'll come down with flu like symptoms and nausea.

>Chocolate makes dogs incredibly sick as well, while not having the same effect on humans.

Few people know this, but the thing in chocolate that kills dogs, theobromine, acts as a caffeine-like stimulant in human beings but with a much longer half-life. Although human livers are much more able to deal with it than dogs, this could conceivably be causing human problems in people who often eat a lot of chocolate less than seven hours before bed.

[0] https://en.wikipedia.org/wiki/Xanthine [1] https://en.wikipedia.org/wiki/Theobromine

>I don't know enough about airborne PFAS to know if it warrants a blanket ban on any products with Teflon.

If you look at the CDC website, PFAS has a rap sheet a mile long. You should probably just clean your pans. If you can afford it, get a reverse osmosis system [3], they are very good at removing PFAS.

[2] https://www.atsdr.cdc.gov/pfas/health-effects/index.html [3] https://www.epa.gov/sciencematters/reducing-pfas-drinking-wa...

The rap sheet you've posted seems to suport my general idea. It states that PFAS has been dangerous in lab animal trials using levels of PFAS that are much higher than environmental levels.

It seems there have been some investigations in humans, but to be honest the effects seem largely similar to a lot of other legal substances. Alcohol is dangerous to pregnant women and infants, a fatty burger can raise cholesterol, etc.

That is a cause for concern, and I've conceded this in my original comment. However, it's not the strongest evidence for a blanket ban.

Or, alternatively, we could treat this as strong enough evidence of an effect and ban many other substances that we use every single day. I can think of plenty of foods that raise cholesterol, and I can certainly think of plenty that pregnant women aren't allowed to eat because of low birth weight fears or other complications.

It's not a question of whether to ban it - none of us could if we wanted to - but of whether to buy it.

> this could conceivably be causing human problems in people who often eat a lot of chocolate less than seven hours before bed

Well so will the actual caffeine.

Still, "not having the same effect on humans" is pretty misleading. It's maybe 4x more potent in a dog. Body weight is a much bigger factor. Not many people are going to eat "one ounce of milk chocolate per pound of body weight" or the equivalent dose of darker chocolate.

This of course varies by person.

I am more or less immune to caffeine (in human concentrations), drinking all kind of coffees before going to bed (espresso, filtered, turkish, ...).

I noticed several times that it had the opposite effect (putting me to sleep). My brother has the same.

Only because it's too expensive

1 oz/lb is about 5kg for an adult male. The price isn’t the only reason you’re not going to eat that much.

Xylitol the artificial sweetener is another thing that is harmless in people but very bad for dogs.


except: "Excessive consumption may induce laxative effects"

I wonder about this. Is this a benign side effect, or the body's way of moving a troublesome substance out of the digestive system?

> Alcohol kills humans in relatively small quantities as well

In absolute quantities it's less toxic than fructose, paracetamol or THC. It's among least toxic substances found in nature. Almost no one dies from acute toxicity of alcohol - almost all alcohol-related deaths are due to accidents and chronic toxicity (long term exposure to toxicant).

I don't know of any household appliances that as part of their regular functioning doses you with alcohol.

Or many household appliances that can reliably kill of your pets at all.

So I don't really get this nitpick.

The point is that quantities of alcohol that hurt/kill birds aren't illegal, because those quantities aren't dangerous enough to humans to warrant legal action. It's similar to how we don't accept animal trials as all the evidence required to approve a new drug.

Some things kill animals and not humans. Some quantities of substances kill animals and not humans. Knowing that a product kills your bird is cause for investigation or concern, but not a reason for a blanket ban.

Also, they use warfarin to treat stroke victims and rat poison. Because it is poisonous isn't reason to not use it in humans.

> Some ovens are teflon coated, and some of those have cleaning modes that effectively kill of pet birds.

it seems unlikely that they have both teflon coat it and a self-cleaning mode (800+F) that burns it off?

Dihydrogen monoxide kills 3.9k in the US every year and can kill pretty much every living thing if consumed in large enough quantities. It's crazy that we consume the stuff on a daily basis.

I see wet you did there...

To me, this indicates that FDA testing / validation needs to be heavily modified.

Teflon is 100%, perfectly safe, as long as it never overheats. It is completely inert, causes no problems, can be eaten, etc.

As you say, when it overheats? It becomes toxic. I read some of the FDA docs referenced during approval, and it was along the lines of "As long as overheating does not occur".

They knew.

But then ran with the premise of "Citizens won't accidentally overheat their pan!". It really only takes a second! A pan on the stove, a distraction (kids, phone, a loud noise outside, whatever), a simple mistake, and BAM pan is overheated.

I'd say every teflon pan in use is toxic, essentially.

I don't know if the approval process has changed or not, but it seems like depending upon people to have 100% perfect adherence to pan heat levels is a bit much...

What other substances are approved this way? Frankly, I think any non-stick substance is suspicious, and use stainless steel.

The trick? Never use soap to wash it. Just use water, soak if needed, and if you can't get it all off? The next cooking will just be all the tastier. :P

Let the grease soak in, let the oils soak in, it does season a bit.

> It really only takes a second!


You don't have to exaggerate that much.

> I'd say every teflon pan in use is toxic, essentially.

Do you find that oil in your pans keeps catching on fire?

Even if you do, a little bit of oil can give you a nice warning to prevent teflon-damaging levels of heat.

Once gasses have been released, the chemical structure of telfon has changed. Literally, this is how it becomes toxic.

It takes very little time, just one little mistake, and that pan is now no longer inert teflon, but instead toxic teflon. Some have suggested that teflon should literally be banned, as a result of this.

Some even regularly cook at temperatures which are dangerous for teflon, eg > 600F, when searing a steak for example.

But I guess my whole point is, we 100% know it can become toxic. We also know that mistakes during cooking, or people not being aware, can cause this transformation. And it's a thing we put our food in, for crying out loud!

I don't think one can overstate how ... wrong it is, to use any non-stick coated pans. It's just not worth it.

Another one is storing pans in the oven, then forgetting the pans are there and preheating it.


Well, sorry, but PAHs are toxic and cause cancer - there are lots of toxins involved in the process of cooking. The food itself is ridden with AGEs, which are toxic and impossible to get rid - "thank" them for diabetes, dark spots, macular degeneration, aging in general, and all kinds of chronic disease.

So you contend that there are other dangers, so why not just add more?

What bizarre logic.

Not really - all these have the same solution, and it is better ventilation; much better, indeed.

The actual teflon, the coating becomes toxic. Permanently.

It is an inert, non-organic reactive substance prior to being damaged. After, its molecular structure changes, it is bio-reactive and toxic.

So no, the solution isn't better ventilation, even for some of the questionable things you mentioned. And yes, you attitude before was "meh, gonna happen anyway".

Oh, it surely is. And cooking methods are source of toxins, too! Most cooked food is rich in exogenous AGEs, and do accumulate just like the endogenous do, and there's no known way to clean them up. Pancreatic cancer is blamed on burnt red meat, but people just like eating toxic food. Cooking in pans, teflon-coated or ceramic-coated is toxic anyway, but, I agree, to different degrees. Stainless steel cookware loads you with iron, which also has an upper limit beyond, which it becomes toxic and generates ROS.

Canary in the coal mine...?

Last week, on Last Week Tonight, John Oliver talks about PFAS, which was very interesting. Not only teflon, but all kind of repellent fabrics like Gore-tex:


I’m not saying he’s wrong — I have neither the education nor the experience nor the time to do so — but as a word of caution: One single atom in a different place, or a chiral flip of the molecule as a whole, can radically change the biological response of a chemical.

H2O is good; remove one atom, get explosive H2; add one atom get the bleach H2O2.

For chirality: S-penicillamine is an anti-arthritic, R-penicillamine is highly toxic[0] (I was going to use thalidomide as my example, but that turns out to be something which changes between both chiralities inside your own body).

[0] https://en.wikipedia.org/wiki/Penicillamine)

If you're worried about goretex, switch to Columbia outdry, it doesn't use or rely on dwr so I'm guessing whatever plastics its using is pretty stable.

If you’re in the Bay Area, you can stop by Fort Funston and see humans using this effect as well.

In the hang gliding world, ridge lift is a wonderful phenomenon that enables one to stay airborne almost indefinitely, with much better predictability than thermal soaring. The lift band is well defined by the incoming wind and upward deflection. When you’re done, just turn inland and exit the lift band.

Same thing at Torrey Pines, if you are in San Diego.

Another interesting thing that links with the latter part of the article - this was exactly why canaries were used in coal mines. The lungs of birds rapidly absorb anything in the air, meaning they'd die before the coal miners.

The main reason was to protect against carbon monoxide. Humans do not feel it and there is no body alert, like we have for carbon dioxide for instance.

> like we have for carbon dioxide for instance.

Carbon dioxide is the only substance for which we have such an alert. (Although there is another kind of alert for substances which smell bad. Most toxic substances don't smell bad.)

Which is why it's horrifying that CO2 asphyxiation is the "euthanasia" method of choice for everything from lab animals to livestock. We've managed to select the one gas that inflicts excruciating pain.

At a guess, I'd say we selected that gas so that, in the event of a safety failure, human operators would notice they couldn't breathe, and flee. It's a pretty good reason.

Assuming the humans are always in a ventilated area, though, nitrogen seems better along several dimensions.

Protip: Get some carbon steel pans (can't go wrong with de Buyer brand) or good old cast iron.

Though, even with teflon gone from your kitchen, you've still got to be careful with other toxic fume sources such as overheated oil or the wax coating that most new pans come with :)

As for the pans: Just use them as you would any other pan and after a while you'll find them to be just as non-stick as any one of those overpriced poison pans. Don't bother with flaxseed oil as people on the internet like to recommend, soapy water is fine as long as you dry the pans well afterwards, and if you want to use metal cooking utensils in your metal pan don't let anyone stop you.

Helicopters have landed on the top of Mt Everest, more than once.

True that not every helicopter can do it, but it can be done.

Yes. Curiously, the pilot reported that they also used updrafts.

I thought the problem was that in the thin air, the rotors have to spin much faster and the tips go supersonic.

Would rotors with large surface area be how those helicopters work?

Oil alone in roughly the same temperature range has the same effect on birds.

That being said, I have nitrided, stainless steel and cast iron cookware despite not owning a bird.

Where do you find nitrided pans?

I only see skillets on Amazon, but I lucked into a $50 wok at Asian Family Market in Seattle. Sorry I can't remember the company name, no English, there was a picture of a chef on it.

I've seen them at marshalls (US) before

> As any parrot owner knows, if you overheat a teflon pan, all the birds inside your house will die.

How about the newer ceramic ones? I assume nothing will happen?

Anodized aluminum would probably be safe also.

I try to tell everyone I know that cooks to buy anodized aluminum. No "seasoning", no scratching, very non-stick, heats rapidly, dishwasher safe, light weight. It's like a miracle material but it's just aluminum.

Great stuff, but it doesn't work on induction stoves. If you learn to cook on stainless steel, it's really great for cooking.

Lots of hard anodized aluminium pans are made to work with induction. Quick Google search: https://www.tefal.com/Cookware-%26-Kitchenware/Pots-%26-Pans....

Perhaps some are fitted an iron plate under, make them work with induction.

The best pans I have are tri-ply, aluminum in the middle with stainless steel on the inside and outside.

supposedly most anodized alum pans are coated with a a nonstick coating (probably teflon/derivatives)

If you put aluminium in a dishwasher enough times, it will gradually eat it away.

What's it anodized with?

What do you mean by that question? It can only be anodised with an electric current by definition, but I assume you have some other concern in mind?

How about DLC?

efficiency and fragility seem to go together (see our current global container shipping fiasco and our just-in-time production world)

The life at the deepest points of the ocean also seem quite fragile.

Also, "canary in the coal mine". Canaries were sent into coal mines to detect carbon monoxide and other gases because they were more sensitive than humans. The bird would die before humans, giving an early warning.

> thinking how helicopters can't fly over everest.

A specially built helicopter did land at the summit.

[0] https://youtu.be/iu46fJDUvog

TIL Richard Stallman uses steel pans.

If you remove the word 'can' from the title, you get another interesting question, and some scientists think the answer is that the bar-headed geese have been doing it since before there was a mountain range there!

> bar-headed geese have been doing it since before there was a mountain range there

The Himalayas formed fifty millions years ago. Geese apparently appeared ten million years ago. How would this be possible?

They weren't nearly that tall then though. E.g.:

> “The bar-heads have done that migration for millions of years before the Himalayas were as tall as they are now, and the birds have been pushed as the mountains have moved up to go higher and higher,” says coauthor Julia York, now a graduate student at the University of Texas at Austin, in an interview with the Times. York, who played foster parent to seven geese, adds, “They’re amazing athletes.”


The mountain range may have been significantly less tall back then but I'm quite sure it had already existed before any species of geese appeared.

It wasn't that particular species at the time but a precursor.

Your grandad is/was hom. sap. sap. but your (50,000,000 / 20)th grandad was one of quite a few possibilities, mostly ape and no sign of homo. I'm using a simple 20 year per generation model here. That's 2,500,000 generations ago. Here in the UK on telly, a comedian was shown how his (2021 - 1239) / 20 = 39 or so generational ancestor was English King Edward I - "longshanks". 39 generations gets you to 1230 or so, 2,500,000 generations is quite a long time ago.

So, let's look at these geese. 50M years back, the "dinosaurs" flying that route - let's call them "Goosey" - already had phenomenally efficient lungs compared to say mammals. As the land rose, they simply adapted a bit. Given how efficient their lungs are I doubt there was much to do.

It looks miraculous to us but we don't have those lungs. We didn't evolve the double ended lungs and hollow bones etc because we simply didn't need them or the cost of some part of that mix was detrimental for our use case or "we" simply rolled a double three. Evolution is weird. Well, no it isn't, that implies a blind watchmaker/intelligent design or nonsense as I prefer to call it.

In the words of a famous bloke: "Evolution is as evolution does". No more and no less. There are some geese that are able to fly over some mountains.

The claim specifically said "bar-headed geese", though. That's a concrete species, not just some random evolutionary ancestor.

> 50M years back, the "dinosaurs" flying that route - let's call them "Goosey" - already had phenomenally efficient lungs compared to say mammals. As the land rose, they simply adapted a bit. Given how efficient their lungs are I doubt there was much to do. ... It looks miraculous to us but we don't have those lungs. We didn't evolve the double ended lungs and hollow bones etc because we simply didn't need them or the cost of some part of that mix was detrimental for our use case or "we" simply rolled a double three.

Are you saying that the Himalayas drove the development of birds world-wide? But despite that, we can only observe it in one species today? That makes even less sense to me.

If that's true they've also been pigheaded since before there were pigs which probably explains the name.

tldr bird respiratory system is very effective, they breathe in one end of their lungs and out the other, always with either a full breath in their lungs or two breaths in their air sacs (one fresh, one ready to be exhaled)


We already have artificial continuous flow hearts, next up: continuous flow lungs.

Some people have (kinda) learnt to - https://en.wikipedia.org/wiki/Circular_breathing

That's very different. Birds have a full lung load of air stored in sacs and bones and that fills the lungs as the stale air is expelled.

  Mammal: Inhale -> burn -> exhale
  Bird:   Inhale -> burn 
                 -> inhale -> exhale 
I think I've over simplified the bird method but basically they inhale and exhale at the same time as required. They literally have double ended lungs, you push air in at one end and exhale CO2 at the other end. We mammals use the same route in and out of our lungs and the whole thing is driven by our diaphragm which pumps the bottom of our pleural cavity.

The bird mechanism is obviously efficient for oxygenation but it must have a cost that our body plan discarded or at least failed to even consider many millennia ago.

(Am I the only one who have read this as a Haskell function declaration, and the signature just didn't make any sense?)

> ... it must have a cost that our body plan discarded ...

Our body plan didn't discard this mechanism, any more than it discarded wings or beans. We never had any of those features in our ancestry, because mammals aren't descended from birds. Our most recent common ancestor is much, much earlier than bird ancestors began evolving any mechanisms related to flight or high-altitude breathing.

> but it must have a cost that our body plan discarded or at least failed to even consider many millennia ago.

You only climb mount improbable, you don’t go down it

I guess the main drawback of such a solution is coughing up stuff? Probably harder to make air go the other way?

Is this 'just' continuously blowing out from air stored in cheeks, while breathing as normal through the nose? I have said 'just', but it doesn't seem easy! It doesn't seem to me that the lungs are being used differently (to normal breathing). I could be wrong.

(Edited to add text in parenthesis in 3rd sentence)

Yes, take a glass of water and fill your mouth, pretend to be a fountain spitting a stream of water, and breath through your nose. That is how I was tought to learn it anyways.

Like a literal jet engine. Continuous flow of air

1) jet engines are interesting because of the continuous flow and also that all the rotating parts rotate in the same direction, a huge advantage over reciprocating engines. So jet engines can do 50,000 rps, while piston under 20,000 rps.

Note that sleeve piston engines like the Rolls Royce Crecy, are much more efficient than valves, but development stopped at the end of WW2 and resources focused on jet engines.

Rolls Royce Crecy - The Most Advanced Piston Aero Engine Never Made


2) the exception to continuous flow is the WW2 V-1 pulsejet engine, which has a gate that is closed at combustion time, about 42 Hz.

The pulsejet is a very interesting engine in that it's the simplest possible jet engine - literally an empty metal tube with spray nozzles and a sparkplug.

The V-1 was the first mass-produced cruise missile. The Germans also had air-to-ground (anti-ship) guided missiles both wire-guided and radio-guided with a TV screen(!)

But the US Navy was the first to build precision-guided autonomous missiles, both the all-analog Sidewinder and Wall-eye, which shared modules. The Wall-eye made the famous Gulf War photos you've seen of entering via windows. It had analog circuitry to do edge-detection in real-time.

I really hope you mean rpm, not rps.

Well this seems interesting... So from my knowledge of high school honors bio, blood vessels have these long slow muscles that also help pump blood. If a heart pumps continuously, wouldn't that create back pressure on the vein/artery muscles, which would in turn increase your blood pressure whenever the vessel muscles contract to pump? And if you got an artificial heart, I'd think higher blood pressure is a bad thing?

From my quick research just now, it seems continuous flow hearts are not better because they pump differently. That's just a side effect with no benefit that's noted in studies. A continuous pump is much smaller, and for long-term total heart replacement, is the only thing that can be small enough to fit in the body. In fact, it's noted in a case study that they're not sure about the long term effects of not having pulses, and that's something that will need to be studied.

Now as far as the lungs - I think that would be a bad idea too. We'd need separate flow-through pathways to inhale and exhale. So two necks, or an exit hole in the chest. That takes up space and is another vector for infection. In addition, exhaling moisturizes the tissue, so you'd need much harsher intake tubes, and your exhale tubes would be constantly dripping water. All that extra space has to come from somewhere - meaning you now have less space for actual oxygenating tissue, resulting in worse oxygen capture. Now the diaphragm has to pump harder, because you're not extracting as much oxygen from your air intake.

Anywise, you had a funny comment, which I hopefully made funnier by responding to it seriously. We're a good team. Team Heart & Lungs they call us.

Yes, Medlife Crisis talks about the issue of continuous flow artifixcal heart and mentioned that work is happening on one than mimmics the heart better in one of his videos.

so when I google "medlife crisis continuous flow heart," the first result is your comment. the one i'm replying to right now. which is pretty funny. do you have a link or good thing to search for, because I can't find it on the 1st couple of pages of results. not "asking for a source" - just bored and looking for something to read.

This is the YT channel https://youtube.com/c/MedlifeCrisis but I‘m not sure which video the GP was referring to.

Yes, I'm not sure in which video he talks about this, it's a fairly recent one.

He's on Twitter and a total medical/heart geek, I'm sure you could ask him and he'll probably provide you with references.

are birds able to take advantage of air speed when flying to get higher air pressure in their lungs?

Apparently swans (and similar) birds have long necks to provide a constant amount of re-breathed air (eg a built-in inefficiency threshold).

Because their lungs are tied to their flight muscles, they would overbreath at cruising speed. So the long necks provide just the right amount of re-breathed CO2 to offset this.

(I hope I have this correct)

Some falcons have nostrils that are designed to slow down the air flow so that they can continue to breathe while diving at very high speeds. Even though they exercise hard to catch their prey they aren’t winded as a result. Unlike say cheetahs that are completely out of breath at the end of a chase and have to breathe for a bit before they can eat..

'pushing the envelope' is actually a term from the bird-racing nerds community for packing those things while staying aerodynamic

The author seems to pinpoint the cause of the great Permian-Triassic extinction to the acquired ability of microorganisms to process lignin, while most of the researchers attribute it to the extensive Siberian Traps eruptions.

Granted, decaying lignin subtracts oxygen from the atmosphere, but had that happened alone the levels of oxygen would have gone down quite slowly. What we see instead is a drop from ~30% to 12% within a few hundreds of thousands of years. It means that a catastrophic event (such as prolonged supervolcanic eruptions) probably played a bigger role.

Even if you converted all the atmospheric CO2 to O2, oxygen levels will not increase by more than 1%. So where did the rest come from and go to?

It came from CO2 and other carbon gasses. IIRC, according to current theory, there were 3 major climates in the earth. In the first climate, the atmosphere was volcanic gasses, many of which were blown off into space, leaving the second climate of carbon gases as a residue, and then this was absorbed into rocks as well as consumed by CO2 breathing lifeforms.


One past thread:

Why Birds Can Fly over Mount Everest - https://news.ycombinator.com/item?id=23639294 - June 2020 (96 comments)

It's not a question of where he grips it! It's a simple question of weight ratios! A five-ounce bird could not carry a one-pound coconut.

African swallows are non-migratory, man.

This is very well written, I now have a story to read to my son this weekend.

On that note, I'd welcome pointers to similar narratives that explain evolution, the formation of the planet, the universe etc.

My father wrote that and he just changed the opening line to include all of his grandchildren, not just his niece as he originally wrote it. I will be reading it to David in the near future. He's only 6. I don't know of any other narratives, but I know he was talking to John Lasseter about Lignin to maybe get the idea into movie form.

Wow, that story is amazing well-written. I would definitely tell it to my kids (if and when I have them).

has the lignin story held up though? Wikipedia's article on Coal suggests that a 2016 paper "refuted" the idea. I don't really follow this area of research, so would be happy if someone more knowledgeable chimed in about it.

I love reading both Nautilus and Quanta. Great articles.

So that's what dinosaur tastes like.

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