In your cellar there are three light switches in the OFF position. Each switch controls 1 of the 3 light bulbs on the floor above. You may move any of the switches, but you may only go upstairs to inspect the bulbs once.
How can you determine which switch operates each bulb?
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The Answer:
Turn switch 1 on for a while... about 5 mins or more. Then turn it off and turn switch 2 on. Go inspect the bulbs.
The hot bulb is controlled by switch 1, the lit bulb is controlled by switch 2 and the unlit and cold bulb is controlled by switch 3.
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