system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 How/Why do calluses form?
gpt
Your body has evolved, on a microscopic level, to adapt to repeated damage by repairing itself to be stronger than it was before.

In the case of callouses, your skin responds things like pressure, friction, tiny abrasions and breakages by building tougher surfaces using keratin (the same protein that your hair and nails are made of).

This sort of action apples to many kinds of cells, including muscle tissue. For example, bodybuilders grow so large by inflicting controlled amounts of damage to their muscle tissues on a precise schedule. Then, they feed their bodies important proteins and amino acids which encourage regrowth. This causes their muscle fibers to heal and grow in order to adapt to this new stress. As far as your cells are concerned, this must be what is necessary to catch and eat food. So we have to adapt to this new normal. If that stress is removed, the muscle tissue reverts back to its previous state. 

Most of the time, this is good, but sometimes repeated stress and damage can cause scar tissue to develop in places you might not want it which can cause long term pain and in some cases increase risks of cancer later in life. Callouses are a pretty mild form of this, some people think they're rough and unsightly, but if you play guitar or work with your hands, those thick callous pads can protect your sensitive nerve endings and soft tissues from damage as you keep on doing the things you love.