system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do cysts form?

I am aware that there are forms of cysts.

So why is it trauma to an area causes the body to create a "sack" instead of just pus (white blood cells?)  in some cases. And if it is a sack, how does it fill up from the inside and then grow and grow to the point it protrudes?
gpt
There are many examples of cysts as you stated, but here are a couple. A bakers cyst forms behind the knee any time there is an injury or condition in the knee causing an inflammatory reaction leading to increased synovial fluid production. The rear of the knee is just the least structurally stout area so that is where the fluid tends to collect. A ganglion cyst usually forms due to a weak spot in a tendon sheath like a weak spot in an inner tube. The fluid in the sheath tends to “inflate” the weak spot, then stagnate and gelatinize forming the cyst. Bones can have an infarct or other condition that can cause an area of medullary bone death that is typically walled off by the body and then a cyst forms in the void. There are many more but these are a few.