You have two containers, one that holds 3 liters and one that holds 5 liters, and access to a water source. Place exactly 4 liters of water into a third container (one that holds at least 5 liters, so that you can't simply fill the container to the brim and be done with it).
The solution is as follows:
First, fill the 3-liter container to the brim.
Pour the 3 liters of water into the 5-liter [...] [tvtropes.org]