You have two jugs. One holds exactly 3 gallons and the other 5 gallons of water. You have unlimited supply of water. How do you fill 4 gallons of water into the five-gallon jug?
Fill up the three-gallon jug and pour it into the five-gallon jug.
Do it again and there will be one gallon left in the three-gallon jug.
Empty the five-gallon jug and pour in the one gallon of water from the three-gallon jug.
Fill the three-gallon jug with water again and pour it into the five-gallon jug. The five-gallon jug should now contain 4 gallons of water.