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認爲，大多數頭盔（用於騎自行車、滑板、滑雪等）內部都有一個堅硬的泡沫，可以皺縮以吸收衝擊力，但一旦皺縮，就不會再反彈了。你的頭盔在經歷了幾次大的跌倒後，最終會停止保護你。