You take the seven-gallon container and you fill it up and you pour the contents of it into the 13-gallon jug. And then you do that again. And when you do that, it's not all going to fit. You'll have filled the 13-gallon container, and you will have one gallon left over in the seven-gallon container. You pour out the 13-gallon container. So now you have one gallon in the seven-gallon container, and the 13-gallon container is empty. You then take that one gallon and you pour it into the 13-gallon container, leaving room for 12 more gallons. You then fill the seven-gallon container again and pour the entire seven gallons into the 13 gallon container. And then you do that again. And when you do that, it's not all going to fit again. You'll have filled the 13-gallon container, and you will have exactly two gallons left over in the seven-gallon container.