Bill invites his family over for dinner one evening. As they sit at the table, the family notice that the plates don't seem very clean. "Pardon me, son," his mother asks. "But have these plates been washed?"

"Why, yes," Bill replies. "They're as clean as soap and water could get them." The family is skeptical but go ahead and eat the dinner; despite the state of the plates, the food is delicious.

After dinner is over, Bill takes the plates and sets them on the floor; he then turns to the doorway, whistles and calls, "Here, Soap! Here, Water!"