system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What exactly happens to muscles when they go "sore" ?
gpt
When you use a muscle to the point of soreness, you are literally tearing the muscle fibers apart. They rebuild themselves and get larger and larger (that's what lifting weights does and why you get bigger muscles) 
But just like a cut on your finger, while being repaired, they are kind of tender, raw and stiff. Your healing from the damage you did.