Measuring 7 Liters Puzzle

Jack and Jill did not have much success when they went up the hill to fetch a pail of water from the well. However, once they had both tumbled down the other side they found the pond and decided to get the water from there instead.

The problem was that they wanted 7 liters exactly. Jack's bucket held 3 liters and Jill's bucket held 5 liters. How did they bring back 7 liters? No guessing!