In one particular night of an unknown season, in an unknown year, three persons choose to bed down in a certain hotel. Their professions are, respectively, mathematics, engineering, and physics.
As luck would have it, fires break out simultaneously in each of their rooms during the night.
The engineer is roused by the smoke and heat, and takes immediate action. Going to her bags she fetches a table of heat of combustion of common substances, and a graduated cylinder. Making the appropriate calculations, she measures out a quantity of water and douses the fire with it. The fire goes out, and the water boils away to leave a dry spot. The engineer returns to bed.
The physicist awakes to find a fire in his room, and proceeds quickly to the bathroom, where he fills a bucket with water. He then douses the fire (and, indeed, much of the room) with the water. The fire goes out, the room in drenched, the physicist goes back to bed.
Now, when the mathematician's sleep is similarly interrupted by a fire in her quarters, she too gets up and proceeds to the bathroom. Once there, she turns on the faucet and runs her hand under the water. Exclaiming to herself, ``Ah! Solution exists!'', she turns off the water and returns to bed.