Answer:
I am black when you buy me, red when you use me. When I turn white, you know it's time to throw me away. What am I?
I climb higher as I get hotter. I can never escape from my crystal cage.
I can be thin but not fat; in your body but never on your placemat. I'm always better when I'm fresh but you'll never see me in the flesh.
I can sizzle like bacon. I am made from an egg. I have plenty of backbone but lack a good leg. I peel layers like onions but still remain whole. I can be long like a flagpole yet fit in a hole.
What gets sharper the more you use it?
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.