You face this classic puzzle in deductive reasoning:

A closed room contains a light bulb. Outside the room are four toggle switches, all initially off.
Exactly one switch controls the bulb.
You can flip switches however you want, but you’re allowed to enter the room only once to observe the bulb (its on/off state, and whether it’s warm).

Goal: Determine exactly which switch controls the bulb, using only one visit to the room.

Step-by-Step Strategy

You’re allowed to touch the bulb and check whether it’s on or warm. This allows you to gain more information than just sight.

Step 1: Divide and Conquer with Time

Turn on Switch 1 — leave it on for a few minutes (say 5).
Turn off Switch 1, then turn on Switch 2.
Leave Switches 3 and 4 off.
Immediately enter the room.

Step 2: Observe and Infer

Once inside, observe the bulb:

Bulb is ON → Controlled by Switch 2
Bulb is OFF but warm → Controlled by Switch 1
Bulb is OFF and cold → Controlled by Switch 3 or 4

Now, you’ve narrowed it down to:

If warm → Switch 1
If on → Switch 2
If off and cold → Must be either Switch 3 or 4

Step 3: Handling 3 or 4

Now you only need to distinguish between Switch 3 and 4. This requires just one more experiment. Repeat the process but only for the two remaining candidates.

However, the puzzle specifically asks for the minimum number of room entries to guarantee the answer.

Final Answer

You only need to enter the room once
With clever timing and heat detection, you can determine exactly which switch controls the bulb.

Reference

[1] [Puzzle 7 (3 Bulbs and 3 Switches)](https://www.geeksforgeeks.org/puzzle-7-3-bulbs-and-3-switches/)