A mathematician, engineer, and a mechanic are sharing an apartment. One night a fire starts in the kitchen. The mathematician wakes up, goes to the kitchen, sees the tap, a glass and thinks ``That'll work'', then goes back to sleep. The engineer wakes up, goes to the kitchen, sees the tap, calculates the size of the glass required to put out the fire, adds 20% for safety, and goes back to sleep.
The mechanic wakes up, sees the fire, runs over and pisses on it to put it out.