Play · Warm-up
Three switches, one bulb
Three switches downstairs. One of them works an old-fashioned bulb in a closed room upstairs. Flip the switches as much as you like, then go upstairs once.
The puzzle
Downstairs
Upstairs
Which switch works the bulb?
How to think about it
Find a hidden signal
Three switches need three different outcomes, but a bulb is only on or off. Look for a second signal. An old bulb that has been on for a while stays hot after you switch it off. Turn one switch on for ten minutes, turn it off, turn another on, then go up.