- https://www.youtube.com/watch?v=NmM9HA2MQGI
 - https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exc...
I also like the prime number factorization explanation. It's a little harder to understand, but it's actually used in RSA which is pretty cool.
There's also an interesting public key encryption system you can build on top of DH called ElGamal. https://en.wikipedia.org/wiki/ElGamal_encryption
What makes ElGamal so interesting, is that it's probabilistic and that you can easily prove its semantic security (from relatively straightforward assumptions), and more importantly 'indistinguishability under Chosen Plaintext Attack' (https://en.wikipedia.org/wiki/Ciphertext_indistinguishabilit...)
Ie even if the attacker can guess your plaintext and knows the public key, they can't be sure that they guessed right.
This was one of the best talks I ever attended. The other was Lynn Conway describing how she applied her study of anthropology to initiating a revolution in VLSI design. I did not know or appreciate at the time how being a trans woman also informed her understanding of the revolutionary process.