1. A physicist, a biologist, and a mathematician are sitting outside a cafe having tea. They watch two people go into a house across the street. After awhile, three people leave the house. The physicist says, "Measurement error." The biologist says, "Simple reproduction." The mathematician says, "If one more person enters the house, it will be empty." --by CW
2. An engineer, a physicist, and a mathematician are all staying at a hotel when for no adequately explored reason a small fire breaks out in each of their rooms. The engineer wakes up, gets a glass of water from the bathroom, and throwing it on the fire puts it out and goes back to sleep. The physicist wakes up, watches the fire for a few moments, measures the area, calculates the approximate rate of burning, works out the optimal amount of water needed, gets it from the bathroom and puts out the fire before going back to sleep. The mathematician wakes up and, upon seeing the fire, puzzles over it for a few moments, does a bit of work and proving that it can be put out goes back to sleep. - by Belgand
3. A dairy farm has fallen on hard times. They invite a physicist to come brainstorm ideas to improve the facility. The physicist spends a couple of days looking around the farm and prepares a presentation for the owners. His presentation starts with "First, let us assume the cow is spherical..." - by Teppo
I stole all those from a discussion in AV Club Onion about Slumdog Millionaire. Good flick by the way!