The light bulb.

In the cellar you have three light switches, one of which turns the light on in the attic.

You can only go upstairs once to check the light in the attic.

Which switch turns on the light?

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 You open the 1st switch and you wait for a while, say 1 min. Then you close the 1st switch, and you open the second. Then you go to the attic. If the bulb is on then the correct switch is the 2nd. If the dulb is off then you touch it.If the bulb is hot then you have turned on the buld, so the oret switch is the 1st. If the bulb is cold then tou have never turn it on, so the correct swith is the 3rd. Of course, all of that are correct if the bulb was turned off from the begging!

 Quote by Rainbow Child You open the 1st switch and you wait for a while, say 1 min. Then you close the 1st switch, and you open the second. Then you go to the attic. If the bulb is on then the correct switch is the 2nd. If the dulb is off then you touch it.If the bulb is hot then you have turned on the buld, so the oret switch is the 1st. If the bulb is cold then tou have never turn it on, so the correct swith is the 3rd. Of course, all of that are correct if the bulb was turned off from the begging!
Are you drunk, or are you typing in the dark?

The light bulb.

I was typing drunk, in the dark!

 Surprising that you are in the dark since you are handy with light switches.
 Flick one switch on and off for about 5 years. Turn it off. Turn on another. Go up into the attic. The one with the black spot is the switch you were flipping, the other two are known due to the one that is on.