Here's a simple problem to exercise your mind. You have two jugs. One holds three gallons and the other holds five gallons. There are no markings on the jugs except a full mark. Your task is to put precisely four gallons of water in the five-gallon jug using only the two jugs. You have an unlimited supply of water available. Describe how you would accomplish this in a comment. There are at least two ways to do it. Don't cheat by looking it up on the internet.

I got this from an old movie but different forms of this problem have been around for ages. The characters in the movie solved the problem but didn't say how -- at least not in the version I saw. You get extra credit if you can name the movie.