Thomas Edison had a beautiful summer house in which he took great pride. One day he was showing his guests about, pointing out the various labour-saving devices on the premises. Turning back towards the house it was necessary to pass through a turnstile which led onto the main path. The guests soon found out that it took considerable force to get through this device.
"Mr Edison," asked one of his guests, "how is it that with all these wonderful modern things around, you still maintain such a heavy turnstile?"
Said Edison, his eyes lighting up with laughter, "Well, you see, everyone who pushes the turnstile around, pumps eight gallons of water into the tank on my roof."
愛迪生有一幢夏季別墅，美輪美奐，引以自豪。有一天，他帶着一些賓客四處參觀，一一介紹別墅之內各種節省勞力設備，然後掉頭走回房子去，途中須經過攔着主徑的十字轉杆。賓客發覺，要走過去，得使勁才能把轉杆推動。
一個賓客說：「愛迪生先生，你這裏處處都是奇妙的現代設備，怎麼卻還使用這樣沉重的十字轉杆？」
愛迪生兩眼露出笑意，回答說：「啊，凡推杆而過者，都給我屋頂的水槽泵入八加侖水。」