I didn’t know there was such a description, but this is what Wikipedia labels as the hardest logic puzzle ever:
Three gods A, B, and C are called, in some order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter. Your task is to determine the identities of A, B, and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for yes and no are ‘da’ and ‘ja’, in some order. You do not know which word means which.
The puzzle, then, is, what’s the fewest number of questions you can ask to find out this information, and what are they?
The article says the puzzle was invented by Raymond Smullyan, which makes sense since he came up with the whole concept of a land of “knights” and “knaves”, where knights always tell the truth and knaves always lie, and you have to try to get some useful information out of them. I used to read a lot of his puzzle books when I was young.
I actually first heard this question at a job interview just a few months ago, at a small startup (no, it wasn’t Google). I mumbled my way through an answer that I think was on the right track, but nowhere near the actual solution (I got it down to about six questions, which is far from the best answer). I never heard back from them. That was actually before I discovered this article. But, if nothing else, at least I know I got stumped by “the hardest”.
If you figure it out, I know where you can go interview.