Why do light bulbs go poof when you turn them on, never while they're turned on and burning and having been on for hours? Maybe some expire in use in an abandoned garage, but I've never seen a bulb give up the ghost unless it happens just as I am turning the switch on. Strange.
-- PM, La Jolla
As your light bulb filament glows away in its glassy prison, minuscule bit of metal fly off it and deposit themselves on the inside of the bulb. The filament gets thinner and weaker as it burns. Finally one day you flip the switch, the sudden surge of current is too much for it to bear, and its little brains explode. Listen closely next time. Maybe you'll hear a tiny scream. Very rarely you might get a bulb that is not completely sealed, and it will burn out the first time it's turned on because oxygen has gotten inside it, causing the filament to burn too hot to survive. In either event, you're still stuck trying to decide which fixture to steal the replacement from.