system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 why do our bodies shed dead flesh all the time yet keep some dead flesh in specific spots i.e. calluses?
gpt
Calluses are formed when there's repeated, mild trauma to skin. "Trauma" sounds dramatic, but any destructive action is considered traumatic. Let's look at the bottom of your feet, for example.

When you get a more severe injury like a cut, your body forms special scar tissues to protect from further injury; these tissues are kind of like skin, but stronger. You probably walk on your feet, and the weight of your body as well as the slight "grinding" motion is slightly traumatic to the skin. Calluses are similar to scars: when you repeat this tiny injury over and over again, like you do when walking, your body makes new protective tissues to protect from those injuries.

There are a couple reasons calluses don't just fall off like normal dead skin, and the most important one is... they do. Have you ever switched to a pair of shoes that fit better, and seen your calluses eventually go away? It doesn't seem like they do when you're still doing the traumatic action, because your skin *continuously* works to maintain a thicker layer of skin that has a higher concentration of super-strong cells called keratinocytes. After the source of trauma goes away, your skin will eventually stop overproducing cells and return to its normal thickness.