Tell a mathmatician, a physicist and a biologis that the empty house in front of them has only one door. Then have two people walk into the house, and later on three walk out of it. They might respond thusly:

Biologist: "They had a baby".
Physicist: "No. There has to be something wrong with the data. A secret door, maybee."
Mathmatician: "No. The house can't be empty, unless somebody walks into it!"