Brain Teazer Answer

You have two containers, one 9 gallon and one 4 gallon, and an unlimited amount of water. Neither container has any markings at all.
Your challenge is to fill the larger container with exactly 6 gallons of water.

1. Completely fill the 9 gallon container.
2. Fill the 4 gallon container with water from the 9 gallon container, leaving 5 gallons in the 9 gallon container.
3. Empty the 4 gallon container, and again fill it with water from the 9 gallon container, this time leaving 1 gallon of water in the 9 gallon container.
4. Dump out the 4 gallon container, and pour the 1 gallon of water remaining in the 9 gallon container into the empty 4 gallon container. The 9 gallon container is now empty. The 4 gallon container has 3 gallons of capacity remaining.
5. Fill the 9 gallon container to its brim.
6. Fill the 4 gallon container with water from the 9 gallon container.
   9 - (4 - 1) = 6