Eli5: Why do we get a bump on our head when we bang it against something but nowhere else on the body?

Question

Eli5: Why do we get a bump on our head when we bang it against something but nowhere else on the body?

In: Biology

Answer 1

First, most other places in the body are soft and that softness elsewhere lessens the impact of an injury at that site.

Second, the swelling on a scalp is forced out from the surface due to the hardness of the bone. Like a ball sitting on a table rather than a ball floating in a pond.