A woman has just purchased an old house, complete with a wine cellar. The cellar is at the bottom of a steep, twisting staircase. At the top of the stairs is a set of three light switches, one of which controls a single incandescent light bulb hanging down in the cellar. Unfortunately, the woman neglected to inquire which switch controls the bulb, and now she would like to find out. The light in the cellar is not visible from the top of the staircase. The woman thinks of a clever way to determine which switch controls the light, while only making one trip down the staircase. She does not use mirrors or any other tricks. What is her method?