You are right next to a river and have a 5 gallon container and a 3 gallon container. You need to measure out 4 gallons of water. How will you do it?
You will fill up the 3 gallon container and pour the 3 gallons into the 5 gallon container. You will fill the 3 gallon container back up, and pour it into the 5 gallon container. You can empty the 5 gallon container and pour the 1 gallon left in the 3