I'm currently reading a book that includes some firsthand accounts from Vietnam-era POWs describing how they killed literally years worth of time in near-total silence and without any reading material. Several mentioned that they enjoyed passing around complicated riddles to keep their minds occupied. This one almost had me resorting to Google, although eventually I got it. Take a shot at answering it in the comments if you want.

You're walking down a road that splits into two paths. One path is safe and leads to your destination. The other is so dangerous that you'll be killed if you take it. You don't know which is which. Each path has a sentry standing watch; one sentry always lies and the other always tells the truth. Again, you don't know which is the liar and which one is honest. You can only ask one question and only one of the sentries will answer it. What's the one question to ask that will guarantee you end up taking the safe path?

I know, right?