Three Light Switches

[Blossie]

I heard on the radio last night this puzzle:

You are standing in the hall of a house and on the wall are three light switches.  You can only switch one light switch on and it must be to the light in the loft.  The other two are disconnected.  You can't see the light in the loft, and you are only allowed to go up the stairs once.  How do you know which light switch is the one that turns the loft light on?

Can anyone help me on this?

[Kathbad]

Turn two on and leave them for a couple of minutes, then turn one of those off and you have 3 possible states upstairs:

1) off and cold

2) off and hot

3) on

These will match up to the switches thus:

1) not been switched on

2) switched on and off again

3) switched on and left on

Thanks

Katherine