You have two perfectly cylindrical containers (empty). One has a 5 gallon capacity (to the brim). The other has a 3 gallon capacity. A huge cask of wine with a spigot is nearby. None of the containers have any markings on them, and you don't have anything to mark them with. The spigot doesn't have any measurement device (flow regulator) to know how much is coming out.

1. Measure exactly 4 gallons into your containers. You may pour wine back into the cask if you need to.
2. Starting with empty containers, measure exactly 4 gallons into your containers. But this time you may not pour any wine back into the cask (or down the drain, etc.). In fact, you may not pour more than 4 gallons into your containers.