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!

Alternate answer:

Fill the 3-liter jug, then pour it into the 5-liter jug. The five-liter jug now has 2 liters of air in it. Fill the 3-liter jug, then pour it into the 5-liter jug until it's filled. You now have 1 liter of water remaining in the 3-liter jug. Empty the 5-liter jug, the pour the 3-liter jug's contents into it. Fill the 3-liter again, then pour it into the 5-liter.

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