Edison's humorous story

The famous inventor thomas A Edison owned a house in the country.
He liked to show things in his house to visitors.
He had made many clever inventions and enjoyed talking about them.
At one place in the garden, he had put a heavy turnstile.
It was a special kind of gate with arms that a person had to push in order to pass through.
Every visitor had to move these heavy arms while walking through Edison's garden.
"You have so many clever things around."
One visitor said, " Why do you have such a heavy turnstile?"
"Well, you see." Edison replied, "Everyone who goes through the turnstile pumps thirty liters of water into the tank on my roof."