You are in a house with two rooms; a downstairs and an upstairs room.
There are three switches in the downstairs room. A, B and C.
There is a single light in the upstairs room.
Only one of the switches will turn on the light.
How do you determine which switch turns on the light, bearing the following in mind:
* There is no direct line of sight between the two rooms.
* You can only go up once and check if the light is own.
* There are no windows in any of the two rooms.