system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does bumping your forehead on something typically results in a bump while getting hit in the arm results in a bruise?
gpt
It's because your skull is so close to your skin.  The blood that comes out of your broken blood vessels has nowhere else to go, so it pools up into a "lump".  

You get a similar lump in any other place where there isn't much flesh between the skin and the bone: your shins or the backside of your forearms, for example.  And when your fingers swell up after an injury, this is the same effect. Fingers are just too small for a proper-shaped "lump" to appear.