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Sitting on the table are 3 light bulbs. 1 of them just blown, 1 of them has been blown for ages, and the other works fine. You don't which is which, but you're allowed to test (turn on) 1 bulb . How can you tell for certain which bulb will work properly.

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Hold your hand over each one. The one that's just blown should still be warm.

Try one of the other 2, and you'll know which one works.

NB: I'm assuming several things you haven't explicitly stated (like "test"ing a bulb just means plugging it in).

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  • $\begingroup$ That's correct! Good job! $\endgroup$ – warspyking Oct 4 '14 at 11:04
  • $\begingroup$ You are also assuming that the one that works also hasn't been used recently, otherwise it would be warm too. $\endgroup$ – Trenin Oct 6 '14 at 16:35
  • $\begingroup$ @Trenin good point, although you can just modifiy the technique. Instead of 1 warm and 2 cold, you have 1 cold and 2 warm. Use the "test" to differentiate between the 2 warm bulbs. $\endgroup$ – TheRubberDuck Oct 8 '14 at 12:39
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    $\begingroup$ Because thermal cycling imposes extra stress on a filament at the moment it's powered on, bulb failures frequently occur less than a second after a bulb is powered on--too little time for the bulb to heat up significantly. $\endgroup$ – supercat Oct 8 '14 at 21:20
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You carefully break open the light bulbs to examine their filaments. The two blown lightbulbs will have broken filaments, and the just-blown one will have smoke inside the bulb.

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    $\begingroup$ It's not necessary to open the lightbulbs, or turn them on. Just shake each buld next to your ear, and you will easily identify the working one. The broken filaments of blown bulbs make a distinctive tinkling sound. $\endgroup$ – ekhumoro Dec 6 '14 at 19:29

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