Riddle #2

Vincent told me another riddle worthy of a blog posting.

Here goes:

A young lady (let's say her name is Janna), buys her first house. On the second floor there are 3 incandescent lights, and in the basement there are three switches. The previous owner told her that each light is controlled by one of the switches, but he could not remember which switch controls which light. So, how can Janna conclusively determine the light to switch relationships with just one trip from the basement to the second floor? (BTW, she can't see the second floor from the basement, it is raining so she can't go outside to look at the second floor, all of her friends are at work, her neighbors are at work, her cat, Merlot, can't speak english so can't tell her which light is working etc etc).