system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what are birthmarks and why do they appear?
gpt
A birthmark is a congenital, 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. Birthmarks are caused by overgrowth of blood vessels, melanocytes, smooth muscle, fat, fibroblasts, or keratinocytes.