Puzzle | 3 Bulbs and 3 Switches

You are standing outside a closed room containing three light bulbs. Outside the room, there are three switches, each connected to exactly one bulb, but you do not know which switch controls which bulb.

You may turn the switches on or off in any combination before entering the room. However, you can enter the room only once, and once inside, you cannot touch the switches again; you can only observe the bulbs.

How will you determine which switch controls which bulb?

Check if you were right - full answer with solution below.

Solution:

• Let the three bulbs be A, B, and C, each controlled by one of the switches (switches 1, 2, and 3).
• Turn on switch 1 and leave it on for 5–10 minutes so that its bulb becomes warm, then turn it off.
• Turn on switch 2 and leave it on.
• Keep switch 3 off at all times.
• Now enter the room and observe the bulbs.
• The ON bulb is controlled by switch 2.
• The bulb that is OFF but warm is controlled by switch 1.
• The bulb that is OFF and cold is controlled by switch 3.

Thus, by using both light and heat, we can identify which switch controls each bulb in a single visit.