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:
Fill up the 5-gallon jug to the top.
Using the 5-gallon jug, fill up the 3-gallon jug.
Empty the 3-gallon jug and pour the 2 gallons left over from the 5-gallon jug into it.
Fill the 5-gallon jug again.
Fill the 3-gallon jug again to the top.

You now have exactly 4 gallons left over in your 5-gallon jug!