system
Given the following post, respond as if you're commenting on Reddit:
human
When black or otherwise colored plastic is bent or cut, why are the stress marks white?
gpt
The plastic doesn't actually change color. Depending on how it's made it happens for different reasons. Plastic is actually crystalline. When you bend it, one of two things happen. You either make lots of little cracks in it, which changes how it refracts light and makes it look white. Or, the crystals in the plastic get stretched out, which makes them absorb and reflect more light, again appearing white.