level 9

You just bought a new house. There is some strange wiring in it. There are 3 switches on the bottom floor. One of them controls the lamp in the main hallway upstairs. You don't know which. You notice that when you came in, all the lights were off, and all the switches were in 1 position (either up or down). You cannot see upstairs, so you can't tell which switch turns turns on the hallway light. (No, there are no windows either)

You can fiddle all you want with the switches. You have 1 chance to go upstairs to see the results.

How do you find out which switch is the one for the hallway?

Ask if you need hints.

Good luck!