system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes Birthmarks?

Are they really caused by birth or something during pregnancy? Or something more sinister?
gpt
Ya know how tattoos are drawn on a deeper layer of skin and therefor never go away?  Well sometimes your body accidentally spills something called melanin which controls how tan your skin is.  It gets spilled on the same layer of skin.