airbag

About a month and a half ago, my airbag light came on. I'm only 16 so I told my parents about it and they told me not to worry about it...

Well about 5 days after it came on, it deployed. I was sitting in a parking lot and I had just started the car. All of a sudden, POOF. It scared the CRAP out of me...

Anyway...Anybody know why that might have happened or how it was set off? The car has never been in any bad accident. It was rear-ended about 3 years ago. Since then, no accidents....

I know there's nothing I can do about it now because of the cost but I'm just curious...