You are in a room in a building, and the building only has 2 rooms. There is no visibility between the two rooms. The room you are in has 3 light switches. The other room has 3 light globes which are operated by the switches in your room.

You have to work out which switch works which globe. You can only go into the other room (the one with the globes) once. You can't cheat by invoking another person and telling them to remember the sequence in which the lights come on.

How do you work it out. Sorry if it is somewhere else on the site, I couldn't find it.