INSIDE A ROOM is a light bulb. Outside are three switches in the off position. There is no way to see into the room without opening the door.
How do you work out which switch turns on the light?
You can fiddle with the switches as much as you like, but you’re only allowed to open the door to the room once.
See the answer
Turn on the first switch, wait a minute, then turn it off. Turn on the second switch and enter the room. If the bulb is on, the second switch is the one that did the job; if the bulb is off but warm, it’s the first switch; if the bulb is off but cold, it must be the third switch that operates it.
