system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What exactly is happening when we pop our knuckles/back/neck?

Why does it feel so good? Is there a difference in popping my knuckles versus popping my back? Why do I get such a good pop on one side and then don't even get a little pop on the other side (thinking of neck and back here)? What is happening?!?
gpt
The joints are cushioned in a sealed sac of synovial fluid which acts as a lubricant to allow smooth movement of the joints without wearing down on each other’s surfaces. 

When you exert force on those joints in a specific way, you’re pulling the joints apart which creates a vacuum like condition on that sac of synovial fluid.  Eventually the fluid cavitates suddenly in a pop which releases nitrogen gas that was dissolved in the fluid.  This bubble of nitrogen gas takes up the volume that was created by the vacuum.  The nitrogen eventually gets slowly reabsorbed into the fluid as the force pulling the cavity apart is removed.  That’s why it takes a few minutes to settle before a joint can pop again.