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!

Here's my answer to this one.

You were going to DO something with those 4 liters of water, right? I don't think you're planning to just leave the 4 liters in the 5-liter jug because it looks pretty. So anyway, fill the 5-liter jug, fill the 3-liter jug from it, and do whatever the heck it is you're doing with the 2 liters remaining. Then do it again.

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.