system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are birthmarks made?

Question sparked by front page picture of man with huge birthmark.
gpt
A birthmark is a benign irregularity on the skin which is present at birth or appears shortly after birth, usually in the first month. They can occur anywhere on the skin. They can be things like excess fat, blood vessels, or skin pigment.