Interesting. I did not even think of that. I don't see many incandescent bulbs these days; the idea that a recently-switched-off light bulb was hot had begun to fade from my memory. I suppose that at the time of the question's heyday, however, your assumption of this second channel of information would have been a reasonable one.
Thanks for sating my curiosity though - I was reading this thread just to see if anyone had an answer to that seemingly impossible question. So that was the answer.
Of course they are relying on you to intuit some property of the situation that they don't mention. If they had said the box has three incandescent bulbs which become hot after they are switched on, most people would "get" the answer easily.
In a real application development situation, however, solving a problem based on what you assume to be true rather than what the customer asked for is usually a mistake.
Thanks for sating my curiosity though - I was reading this thread just to see if anyone had an answer to that seemingly impossible question. So that was the answer.