What is the best interview question ever?

This question was put across me in an interview for an IT firm.

think about it and answer me correctly

I don’t know if it was the best question, but the question actually took me for a surprise.

So the interview started with a warm handshake.

Interviewer: So Eshaan, how much would you rate yourself on a scale of 10 in terms of your reasoning power?

Me: Sir, I would fairly rate myself at 7.5.

Interviewer: That surely is a good score. Actually there is a question that is bothering me from quite sometime now. I was hoping if you could help.

Me: Sure sir ( *smiling as I said that, but inside, his way of framing up the question was making me nervous )

Interviewer: Okay, so there is one room which is totally dark.

It has a bulb holder and a working 100 watt bulb already installed.

The room is closed from all the sides without a window or any ventilator. Except that it has a door without any breathing space below. That means no light from the room can be seen from the outside of the door.

There are three one way switches outside that door. But the bulb inside corresponds to only one of the three switches.

Now suppose that you are on the outside of that room.

The task is to find out that particular switch which controls that bulb.

But you are allowed to enter the room only once.Advane

So tell me Eshaan, how would you solve this problem with your logical reasoning.