It’s an old riddle where a room has three lights and outside the room is the panel with three switches. They’re not labeled and you don’t know which switch controls which bulb. You’re allowed to switch any two, then you get to open the door and you have to determine which switches control which lights.

The solution is, that you flip switch #1, wait five minutes and then flip switch #2. Then you immediately go into the room.

Two lights will be on, meaning the bulb that’s off is the third switch. Then you feel the bulbs that are on: the one that’s warm already is the #1 and the other that’s on but still cool is #2.

LEDs don’t heat up like that so this technique is broken.

“A windowless room contains three identical light fixtures, each containing an identical light bulb or light globe. Each light is connected to one of three switches outside of the room. Each bulb is switched off at present. You are outside the room, and the door is closed. Before opening the door you may play around with the light switches as many times as you like. But once you’ve opened the door, you may no longer touch a switch. After this, you go into the room and examine the lights. How can you tell which switch goes to which light?”

The solution is:

• Turn two switches on, leave one off
• Wait a few minutes
• Turn one of the “on” switches off

Now, when you enter the room, you’ll have one lit bulb, one warm unlit bulb, and one cold unlit bulb, letting you solve the riddle.