In film die hard with vengeance. How do Bruce and Samuel J make 4 gallons in that jug???

There is a scene in the film where Bruce and Samuel J find bomb in childrens park pond. They have 3 jugs. A 3 gallon jug and a 5 gallon jug. They have to get 4 gallons exactly or the bomb will explode!! How do they do it cos I dont get it I think its impossible!!!