The Windowless Room.

Brainteaser Answer 11:

First and foremost, for your own reference, name the switches “a, b and c.”

Then…

Turn on switches a and b.

• Wait 10 minutes.
• Turn switch b off.
• Enter the room.
• Whichever light bulb is now on, is controlled by switch a (the only one, still on).
• Feel the other 2 light bulbs (carefully).
• Whichever light bulb is warm is controlled by switch b.
• The cold light bulb must be controlled by switch c.

All you have to do is think outside the box!