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Why does a lightbulb burn out just as you turn it on and never when it's been on for a while?

-- LLN, La Jolla

As a lighted bulb glows away, minute bits of the metal filament fly off and deposit themselves on the inside of its little glass prison. The filament gets thinner and weaker as it burns until one day you flip the switch, current surges through a wire that's too thin to bear it, and the bulb's little brains explode. If you listen carefully, you can sometimes hear a faint scream.

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