Puzzle of the day: I found this on the PostPolitics website, taken from the most recent issue of Scientific American.

You have a bunch of spies ready to go into enemy territory. When they return to cross the frontier into your country, you want to avoid getting them shot, while at the same time preventing enemy spies from entering. So each must present a password to the guards, which the guards will verify. Whereas you trust your spies, and your guards are loyal, you believe the guards may loosen their tongues in bars at night. What information should the guards receive, and how should the spies present their passwords, so that only your spies get through and nobody else, even if the guards go out for a couple?

(My own proposed solution is here.)