system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come we grow large lumps on our heads if we hit it instead of the skin bruising?
gpt
It's still bruising.  Bruising is when  broken capillaries cause blood to pool under the skin.  In the case of the scalp the skin is so thin that when the blood wells up it bulges the skin out since there's nowhere for it to pool.