He searched his pockets. No more coins; nothing. “I’ll pay you tomorrow,” he told the door. Again he tried the knob. Again it remained locked tight. “What I pay you,” he informed it, “is in the nature of a gratuity; I don’t have to pay you.”

“I think otherwise,” the door said. “Look in the purchase contract you signed when you bought this conapt.”

In his desk drawer he found the contract; since signing it he had found it necessary to refer to the document many times. Sure enough; payment to his door for opening and shutting constituted a mandatory fee. Not a tip.

“You discover I’m right,” the door said. It sounded smug.

— Ubik, Philip K Dick, 1969