GALLON OF GALLONS
Riddle: You have a five-gallon bucket and a three-gallon bucket with as much water as you need, but no other measuring devices. How do you fill the five-gallon bucket with exactly four gallons of water?
Answer: Fill the five-gallon bucket all the way up. Pour it into the three-gallon bucket until it is full. Empty the three-gallon bucket. Pour the remaining two gallons into the three-gallon bucket. Fill the five-gallon bucket all the way up, then finish filling the three-gallon bucket, leaving four gallons in the five-gallon bucket.
Source: https://www.riddles.com/8785
