As part of a research project, a mathematician was asked to take a special intelligence test. The coordinator placed the mathematician alone in a room, empty except for a bucket of water on the floor next to a small pile of twigs.
Almost instantly, the twigs ignited and burst into flames. The mathematician thought for a moment, rushed over, grabbed the bucket of water, and poured it on the twigs to put the flame out. The coordinator then entered the room, complemented the mathematician, and moved her on to stage two of the research task.
Now, the mathematician was placed in a similar empty room, except for a bucket of water and a small pile of twigs. The difference was that now the twigs and the bucket were not next to each other, but on opposite sides of the room along a wall.
Once again, the twigs ignited and burst into flame. The mathematician thought for a moment, smiled, calmly walked over, picked up the bucket of water, and set it next to the burning twigs. Soon the room was filled with smoke and the fire was growing in size, causing the floor to burn. The mathematicians stood there watching contentedly.
Concerned by the mathematician's lack of concern for her safety, the coordinator burst down the door and hauled the mathematician to safety. Once the mathematician had stopped coughing from the smoke and had recovered somewhat, the coordinator asked why she had only moved the bucket and not poured it on the fire as before.
The mathematcian's response: "I wasn't worried. I only had to move the bucket next to the twigs, as then I was facing a problem that I had already solved. Q.E.D."
Source: A.R. (Bellingham)