system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What the heck is a mole exactly?

edit: I apologize for not specifying. I meant mole as pertaining to chemistry.
gpt
Moles occur when cells in the skin grow in a cluster instead of being spread throughout the skin. These cells are called melanocytes, and they make the pigment that gives skin its natural color. Moles may darken after exposure to the sun, during the teen years, and during pregnancy.