Farmer Ron was cleaning out his big old dusty farm shed. He dragged out an old water container which was full of old dirty water and it weighed 5 kilograms. Ron put something in the container and then it weighed less. What did he put in the container?
His head. :D
Could work :O! But the answer I saw was like "a hose connected to a water pump"
That one is less disgusting.
A hole makes more sense.