[nojokes]

A mathematician, physicist and biologist sit in a street café, guarding people leaving and entering the opposite house. They see that 2 people go to the house first. Some time passes and they notice that 3 people are leaving the house and start to discuss about the meaning of this observation. Physicist: "The measurement may have not been accurate enough." The biologist: "They may have reproduced inside." Mathematician: "If exactly one person went into the house now, the house would be empty again."