A physicist, a mathematician and an engineer, are staying in a hotel. At midnight, the engineer smells smoke. He goes out into the hallway and sees a fire. He fills a trash can from his room with water, douses the fire and goes to sleep again.
Later, the physicist wakes up and smells smoke. He opens his door and sees a fire in the hallway. He gets the fire hose and, after calculating the flame velocity, distance, water pressure, trajectory, etc., extinguishes the fire with the minimum amount of water and energy needed.
Finally, the mathematician wakes up and smells smoke. He goes to the hall, sees the fire and then the fire hose. He thinks for a moment and then exclaims, "Ah, a solution exists!" and then goes back to bed and continues his sleep.