The Jugs and the Ocean

You're at the beach and you have two opaque jugs. One holds 13 gallons, the other holds 7. For whatever reason, you need exactly 2 gallons. You have no other containers. How do you do it?

Fill the 7 jug and pour it all into the 13. Fill up the 7 again and pour into the 13 until the 13 is full. You will be left with 1 gallon in the 7 jug. Empty the 13 and pour the 1 gallon from the 7 into the 13. Now repeat the process: fill the 7 and pour it into the 13, then fill the 7 again and pour into the 13 until the 13 is full. Once the 13 is full, you will have 2 gallons left in the 7.