The farmer take his 5 gallon bucket with him to the moonshiner and asks for four gallons of white lightning. The moonshiner only has a 3 gallon jar plus his barrel of unknown quantity. How can he measure 4 gallons with these 3 and 5 gallon jugs?
Fill 5 and use it to fill the three. Leaves 2 in the five. Empty the three into the barrel and then pour the two left in the 5 into the three. Fill the five and use it to finish filling the 3. Leaves 4 in the 5 gallon bucket. ;-)
There are a lot of variations on this puzzle with different sized container and with/without a large source to pour back into. This is the first time that I have seen this method of solving any of them. Kudos to you for that one. ;-)