Reviewing for exams is serious, and therefore one official joke will be provided before each exam.

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 people coming out
of the house.

The Physicist: "The measurement wasn't accurate.".

The Biologist's conclusion: "They have reproduced".

The Mathematician: "If now exactly one person enters the house then
it will be empty again."

An engineer, a physicist, and a mathematician are shown a pasture with a herd of sheep, and told to put them inside the smallest possible amount of fence. The engineer is first. He herds the sheep into a circle and then puts the fence around them, declaring, "A circle will use the least fence for a given area, so this is the best solution." The physicist is next. She creates a circular fence of infinite radius around the sheep, and then draws the fence tight around the herd, declaring, "This will give the smallest circular fence around the herd." The mathematician is last. After giving the problem a little thought, he puts a small fence around himself and then declares, "I define myself to be on the outside!"

Several people are in a hot-air ballon, trying to land over a fog-shrouded contryside at the end of a long day. The balloon dips down low and they see the ground faintly. Spotting a person, one of them calls down: "Where are we?" Some minutes later the wind is carrying them away and they hear faintly, "You're in a balloon!" One person in the balloon gondola says thoughtfully to the other, "It's so nice to get help from a mathematician." The other says, "How do you know that was a mathematician?" The first replies, "There are three reasons: it took a long time get the answer, it was totally correct, and, finally, it was absolutely useless."