Many, many substances ... when they are surrounded by air and get very
hot ... they combine with the oxygen in the air, forming new substances.
The process is called "burning", and the new substances are often called
"ashes".
If the tungsten filament [were] surrounded by air, then when you turned on
the light and the filament got hot, it would burn, turn to ash, and fall to the
bottom of the bulb in a little pile of dust. This would all happen so fast that
you would see a short, bright 'flash', then the light would go out forever, and
you would say "OH ! The bulb burned out. We need to go to the store and
buy another one."
