system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What a callus is and how it is formed.

More specifically on older hands that see lots of work during their lifetime, there are calluses from using their hands so much (I.e. carpenters, woodworkers, etc). What I want to know is, how the calluses come to be and what causes them.
gpt
Your skin is given some atypical stress and is like, "Oh crap, that was pretty tough. Better bulk up in case it happens again."

Your skin starts producing extra keratin, which are basically the cells that make up skin.

The extra keratin piles up, eventually forming your very own callus, tough enough to take on the world!