Three lamps
There are 3 table lamps of similar design in a room, behind closed a door. There is a switch outside of the room that is connected to two of the lamps, so that in the two positions of the switch two different lamps are switched on and the other is off. The third lamp is not connected. You can flip the switch as you wish, but you can enter the room only once.
Can you determine which lamp is not connected?
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that is a very easy one to think.
turn the switch on for a few minutes.then flip the switch and enter into the room.if one of the lamp glowed, it will be still warm as the third one is not connected.