You may have seen this one in Die Hard: With a Vengeance...

You are by a fountain of water with two jugs. One fills up to exactly 3 Liters, while the other fills up to exactly 5 Liters.

A bomb next to the jugs has been activated and is primed to explode. The bomb can only be defused by placing a jug, with EXACTLY 4 liters of water in it, on a scale the bomb is connected to.

Note that these jugs are oddly shaped with no tick marks, so you can't simply fill the 5 liter jug "four fifths" full to get 4 liters.

How can you fill the appropriate jug with EXACTLY 4 liters of water so that you can deactivate the bomb?