A mathematician, a physicist and a salesman are each given 50 dollars to
measure the height of a building. 

The mathematician buys a ruler and a sextant, and by determining the angle
subtended by the building a certain distance away from the base, he
establishes the height of the building. 

The physicist buys a heavy ball and a stopwatch, climbs to the top of the
building and drops the ball. By measuring the time it takes to hit the
bottom, he establishes the height of the building. 

The salesman puts forty dollars into his pocket. By slipping the doorman the
other ten, he establishes the height of the building.