One evening you are sitting in a room with no windows and the door shut. Suddenly you hear two cars drive up outside. From the noise of the engines you can make out that one car's engine is well tuned, whereas the other car's engine is in urgent need of repair. The engines are switched off and you are asked to go out and identify the car with the good engine. You are allowed to start only one engine to hear the noise. However, when you get outside, you find 3 cars. You are told that one of them was already there for a long time.
How can you be sure to find the car with the good engine?
