What am I?

I give life for my own, have a beginning, but my end is unknown. What am I?

related riddles

You do not want me when you don’t have me, but when you have me you don’t want to lose me.

When you are inside me, you will be encouraged to slam other cars. What am I?

Weight in my belly, trees on my back. Nails in my ribs, feet I do lack. What am I?