There is a lightbulb inside a closet which is closed.
You can’t see if the light is on or off through the door.
Though you know that the light is off from the start.
There are 3 light switches outside the closet and one of the switches controls the lightbulb.
You can flip the switches in whatever way you want, but once you open the door, you are not allowed to touch the switches anymore.
Which switch controls the light?
