mediumlogic

There are three light switches outside a room. Inside is a single light bulb, controlled by one of the three switches. You need to determine which switch operates the bulb. You can turn the switches on and off as many times as you wish (they are all off to begin with), but may only enter the room once. There is no one there to help you. The door to the room is closed, and there are no windows, so you cannot see inside. How can you discover which switch operates the bulb?

Do the following: 1. Turn ON two switches, and leave one OFF. 2. Wait a few minutes. 3. Turn one switch from ON to OFF. One is now ON and two are OFF 4. Enter the room. - If the light is ON, it is controlled by the switch you left ON. - If the light bulb is OFF, touch it. If it is warm it is controlled by the switch you turned ON and OFF. If it is cold, it is controlled by the switch you never turned on.

Why this works

This riddle cleverly combines logic with a physical property of light bulbs—heat. By turning on two switches and leaving one off, you create three distinct scenarios when you enter the room. If the bulb is on, you know it's the switch left on. If the bulb is off, you can determine which switch controls it by feeling the bulb; a warm bulb indicates the switch you turned off was the one controlling it, while a cold bulb shows that the switch never turned on is the one responsible. This approach uses both the state of the bulb (on/off) and its temperature to pinpoint the correct switch, making it a fun puzzle that tests both reasoning and observation!

Common Wrong Answers

“Turn on all three switches and enter the room.”

This method does not provide any information about which switch controls the bulb because you cannot differentiate between the states of the bulbs when all switches are on.

“Turn on one switch, wait a few minutes, then enter the room.”

This only tests one switch at a time and does not account for the possibility that the bulb could be controlled by one of the other two switches. You need to test multiple switches to determine which one operates the bulb.

“Turn on two switches and leave the room, then ask someone outside which switch controls the bulb.”

This approach is not viable because there is no one available to provide assistance, and you must determine the bulb's operation independently without outside help.

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