Suppose you have a 3 liter jug and a 5 liter jug (this could also be in gallons). The jugs have no measurement lines on them either. How could you measure exactly 4 liter using only those jugs and as much extra water as you need?
This question was actually asked in a popular movie – Die Hard With A Vengeance.
Try figuring this one out on your own before reading the answer. It should be clear that the goal is to end up with 4 liter in the 5 liter jug, since having 4 liters in the 3 liter jug is impossible.
The answer, broken down in steps: