Fill the three quart jug exactly to the brim, and pour it into the empty five quart vessel.Now refill the three quart jug, and carefully pour as much as will fit into the five quart jug.Exactly one quart remains in the three quart jug.Now dump out the larger jug, and pour the exact quart into it.Fill the three quart jug a final time, and add it to the quart in the larger container.That makes four quarts - one gallon of water!