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?
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. One gallon will be left in the 3-gallon container.