There is an old house with an upstairs room and a downstairs room. the upstairs room contains 3 light bulbs and the downstairs room contains 3 switches. Each switch is connected to one light bulb.
A man is in the downstairs room and needs to know which switch belongs to which light bulb, however the man can only run up the stairs once to check what happened when turning on/off the switches.
How does the man know which switch belongs to witch lightbulb?
Btw the upstairs room/lights can not be seen from downstairs