A physicist, a mathematician and an engineer stay in a hotel.
The engineer is awakened by a smell and gets up to check it. He finds a fire in the hallway, sees a nearby fire extinguisher and after extinguishing it, goes back to bed.
Later that night, the physicist gets up, again because of the smell of fire. He quickly gets up and sees the fire in the hallway. After calculating air pressure, flame temperature and humidity as well as distance to the fire and projected trajectory, he extinguishes the fire with the least amount of fluid.
Then the mathematician awakens, and finds that the embers of the fire are still burning. After giving much thought to the problem, he lights it up to an actual fire. Then he goes back to sleep, satisfied that the problem has been reduced to a previously solved one.