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?