system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why When I hit my leg or head I get a bump there.
gpt
Cells in the area get damaged and release signals that cause the blood vessels in the area to basically leak, like a pipe that gets holes drilled into it will leak water.  This causes fluid (NOT blood) to go out into that area, creating swelling.  This leakage allows immune cells and building blocks to get into the area to help repair the damage and prevent infection (mostly to prevent infection).  The immune cells are able to get through because they have special ways of squeezing through the little tiny holes.

If the blood vessels themselves are damaged (if the pipes themselves are actually cracked open, instead of drilled through), then actual blood leaks out through the holes, including red blood cells, which causes the dark color of a bruise.  The bruise changes color over time because the materials in the red blood cells are broken down.  As they are broken down, they become different materials by stages, each stage has a characteristic color.  That's how a bruise goes from dark purple to greenish to yellow, then back to normal.

Over time all of these extra cells and building blocks (proteins) get re-absorbed, making their way back into the blood vessels in the area.