In a staircase are 3 switches located. On the loft is a turned off light bulb, which can be turned on/off with one of these three switches. The remaining switches don't do anything at all. How can you find out which of the switches is the working one? You can go to the loft only once and you can't see the light bulb from anywhere except the loft itself. You are not allowed to use any devices which might help you.

Post your solution with explanation here!