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Answer to old chestnut: measuring four liters:

First, fill the five-liter jug. Fill the three-liter jug from the five-liter jug, leaving two liters of water in the five-liter jug. Dump out the three-liter jug and pour the two liters of water into it from the five-liter jug. Refill the five-liter jug, then fill the three-liter jug from it. Since the three-liter jug already had two liters of water, only one more is required to fill it, and four liters are left in the five-liter jug.

A common variation of this puzzle also includes an eight-liter jug which is initially full of water. The only difference is that this is used as your water supply and dump; it is simply the smallest integer size that has enough water to solve the problem and doesn't otherwise help!