I encountered this puzzle during my high school at Saraswati Shishu Vidya Mandir, Mukhiguda, where I studied 2005 to 2008. I am not very sure who asked me this, but I guess it was a friend named Sudeep. The question is the following. In a two floor building, a 100W filament bulb is there in a room in the upper floor, whose switch is in a room in the ground floor. But in the switchboard there are 3 switches, among them one is the switch of the bulb, which you need to determine. From the ground floor there is no way to see any light from the room having the bulb. you are given half an hour to stay inside the building, but only once you are allowed to visit the upper floor.