Switch

Lora has an old house in the countryside, which includes a ground
floor and an upper floor.  She decides one day to install
electricity.  An electrician comes and installs three switches on
the ground floor.  Having only one bulb which she installed on the
ceiling of her room located on the upper floor, she decides to
determine which switch is connected to this bulb, knowing that she
does not have any means of seeing it turn on or off from the
ground floor, but that she can turn the three switches on or off
whenever she wants to.  Moreover, Lora is really old and tired,
and her goal is to go up only once to the upper floor.



What trick will Lora use to find the right switch without
climbing the staircase more than once?