Barrel Of Oil
Riddle: You have a barrel of oil, and you need to measure out just one gallon. How do you do this if you only have a three-gallon container and a five-gallon container?
Answer: Fill the 3-gallon container with oil and pour it into the 5-gallon container. Then fill the 3-gallon container again and use it to fill the 5-gallon container the rest of the way. Out of the 3-gallon oil, 2-gallon will be required to fill the 5-gallon container completely. Hence one gallon will be left in the 3-gallon container.
Source: https://www.riddles.com/2602
