system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when your ankles and knees crack?

I recently had surgery on my ankle and have started walking again. I find when I stretch or move it, it cracks and pops a lot more then it ever did before. This doesn’t cause pain, and sometimes even feels nice, but what makes the noise?
gpt
Bubbles popping.

Your joints are encased with a shock-absorbing fluid called synovial fluid. This stuff is non-newtonian - it’s liquid normally to lubricate your joints and reduce friction. But a sudden shock solidifies it to protect your joints.


Synovial fluid can get gas bubbles in it, and applying pressure pops these bubbles.

Surgery on a joint typically includes disrupting the synovial fluid in some way while also modifying the joint itself, leaving the fluid more susceptible to these bubbles until the joint heals.