A variant (I don't have the book in front of me, so this is a paraphrase) of this puzzle was in the book Brainteasers and Mindbenders, by Robert Hamilton. It has stuck in my mind since I read it in junior high or whenever.
A runner is running around a circular track with a radius of t miles. The runner runs around the track a total of n times. Every time the runner completes one mile of his run, he takes a chug of beer, drinking a total of s quarts for every mile he runs. So, given this information...
Prove that the runner only drinks a single quart of beer over the course of his run!