YSK that most helmets (for biking, skateboarding, snowboarding, etc) have a stiff foam inside that crumples to absorb impact, but once it's crumpled, it doesn't bounce back. Your helmet will eventually stop protecting you after a few big falls.
from /r/YouShouldKnow
YSK认为，大多数头盔（用于骑自行车、滑板、滑雪等）内部都有一个坚硬的泡沫，可以皱缩以吸收冲击力，但一旦皱缩，就不会再反弹了。你的头盔在经历了几次大的跌倒后，最终会停止保护你。