This one is a classic (at least inmy country):
In a room (I'll call it the lamproom) there is a lamp.
You're in a room next to the lamproom.
In front of you there are three lightswitches, one of them is used for the lamp in the lamproom.
How can you find out which of the tree switches is for the lamp, when you can go into the lamproom only once?
There is no window or anything trough which you can look into the lamproom.