Three switches. One trip.
A simple room, three identical switches, and one chance to observe. Can you extract enough information?
The challenge
One room. One visit. One correct switch.
You are standing outside a closed room. On the wall are three identical switches—A, B, and C. Inside the room is a single incandescent light bulb. Only one switch controls it.
You may operate the switches in any way you like, but you can enter the room only once. Once you enter, you cannot touch the switches again.
Your task
Determine which switch controls the bulb.
Assume the bulb works normally, begins switched off, and becomes warm after being on for a few minutes.
Before you look
What’s your answer?
Writing down a hypothesis makes the reveal more satisfying—and your reasoning easier to compare.
Your guess stays private. No account required.
I’ll skip the guessAnswer revealed
The key isn’t only light. It’s heat.
The solution is to create two different observable states before entering the room: light and heat.
- Warm the bulb. Turn switch A on and leave it on for several minutes.
- Change the visible state. Turn A off, turn B on, and then enter the room.
- Use two observations. If the bulb is lit, switch B controls it. If it is dark but warm, switch A controls it. If it is dark and cool, switch C controls it.
Why this works
The bulb’s temperature preserves information about its previous state. That gives you a second signal even after switch A is turned off.
Thinking principle: Use more than one observable property.
When one signal is ambiguous, look for another property—temperature, sound, time, vibration, or residue—that preserves information about the system’s previous state.
How did you do?
Your response helps us tune difficulty.