A physicist, a biologist, and a mathematician are at a cafe sitting at an outside table having a cup of coffee. Their attention turns to the woman walking in the front door of the cafe. They watch her open the door, walk in, and close the door behind her. The scientists return to their conversation, but are interrupted shortly when the door opens and the woman walks out with a man at her side.
The physicist remarks, "Our original sample must have been faulty."
The biologist counters, "She must have reproduced while inside."
The mathematician concludes, "Now, if exactly one person walks into the cafe, it will be empty again."
