A Mathematician, a Biologist and a Physicist are sitting in a street cafe watching people going in and coming out of the house on the other side of the street. First they see two people going into the house. Time passes. After a while they notice three persons coming out of the house.
The Physicist: "The measurement wasn't accurate."
The Biologist: "They have reproduced."
The Mathematician: "If now exactly one person enters the house then it will be empty again."
A logician saves the life of a space alien and is rewarded with an offer to answer any question. After a thought he asks: What is the best question to ask and the correct answer to it? After a brief panic the alien consults her computer and says: The best question to ask is the one you just did and the correct answer to it is the one I gave.