system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a blister, and where does all the clear liquid in them come from? Why does friction cause them?

As above. Just started trying to play violin and now have a good old blister the size of a pencil eraser on my thumb. I'm curious as to how blisters actually work.

Thanks reddit!
gpt
A blister is like a watery airbag that pops out of your skin when something hurts it.

That clear liquid is plasma, or blood without the red blood cells and stuff that causes blood to clot. The reason they form is actually a really brilliant feature of the skin.

Assuming it is not a blister resulting from an infection, a blister is essentially your skin creating a cushion to prevent damage. When the skin senses a potentially damaging amount of friction/heat, it reacts by creating a pocket of plasma to protect the area. 

This pocket of plasma does two things. First, it cushions the area by adding an additional, soft layer of protection and insulation. This helps prevent damage to deeper layers of skin. Second, it can be painful, which encourages you to stop doing the thing that's causing it, which helps prevent damage. 

The reason you don't get a blister from something blunt, like getting punched, is because the force of that impact goes *through* the skin. The potential damage that triggers a blister is something happening *to* the skin.