Riddle: You are standing by a river with a 5-gallon jug and a 3-gallon jug, but you need exactly 4 gallons of water. Using only the 2 jugs you have, how can you measure out 4 gallons?
Answer: 1: Fill up the 5-gallon jug to the top.
2: Using the 5-gallon jug, fill up the 3-gallon jug.
3: Empty the 3-gallon jug and pour the 2 gallons left over from the 5-gallon jug into it.
4: Fill the 5-gallon jug again.
5: Fill the 3-gallon jug again to the top.
6: You now have exactly 4 gallons left over in your 5-gallon jug!