You have a 3-gallon jug and a 5-gallon jug. How can you measure exactly 4 gallons of water?
Answer
Fill the 5-gallon jug, pour into 3-gallon jug until full (leaving 2 gallons in 5-gallon jug). Empty the 3-gallon jug, pour the 2 gallons into it. Fill the 5-gallon jug again and pour 1 gallon into the 3-gallon jug (until it's full). This leaves exactly 4 gallons in the 5-gallon jug.