You're standing on the ground floor of a three story building next to a wall on which are three light switches. There is a lamp on the third floor in a windowless room that is controlled by ONE of the three switches. Your life depends on your ability to determine which switch controls the lamp. You're allowed to flip each switch on and off as many times as you like. However, you're only allowed to go upstairs to check the lamp ONE TIME. How do you determine which switch controls the lamp?