Riddle: You have only a 5 gallon jug and a 3 gallon jug. You have to know you have four gallons exactly. You have unlimited water. Both jugs are full. How do you get to four gallons exactly held in one jug?
Answer: There are a few answers but this is only one!:
First you dump out the 3 gallon jug. Second you dump some of the 5 gallon jug's water into the 3 gallon jug so the 3 gallon jug is completly full. Dump out the 3 gallons. Now you have 2 gallons of water held in the 5 gallon jug. Pour the 2 gallons into the 3 gallon jug. Fill up the 5 gallon jug and pour some of the water of the 5 gallons into the 3 gallon container. Now dump out the 3 gallon jug. You now have 4 gallons held in the 5 gallon jug!