Grandfather Jacob turns the first light switch on and waits about five minutes. Then he switches it off again, and turns on the second one. While the second light switch is on, he goes upstairs to see if the lamp is on.
- Yes, the lamp is on. The second light switch is the correct one.
- No, the lamp is not on. If the lamp is still warm, then it is operated by the first light switch.
- No, the lamp is not on. If the lamp is off and cold, the third light switch is the correct one.