system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: how do beauty marks that show up later in life, “form” ?

like I’m 18 and I’m slowly getting a small darkening beauty mark on my cheek that only showed up about a year ago! So what’s the biology of it?
gpt
Cells on your body are constantly replicating and regenerating, they do this *a lot*.

Sometimes, they get it wrong.

The effect of this is often benign, but sometimes can be serious. This is how cancer starts for example. But a mole, freckle, or beauty mark, is not cancer. Hurray!

Basically, what happened is that the cell whose job it is to maintain your skins pigment, called a Melanocyte develops a fault in the instructions it gets when it tries to do its job. Basically what happens when a mole has formed, is that a cell receives different instructions to the other cells around it, begins spewing out a disproportional amount of melanin pigment in the affected area. The more melanin in the vicinity of the faulty cell, manifests as a brown or black mark.

That's all! It's just a cell that has gone wrong somewhere, and is overproducing melanin, making that very specific area of your skin appear darker than the rest.