The cracking sound when you pop your knuckles comes from air bubbles in the joint between the bones.
Usually the air trapped in the cushioning fluid sacs between two bones dissolves.
When the knuckle joint is “cracked,” one bone is pushed out of or into a joint, letting a little pocket of air in.
The jury is still out on whether cracking your knuckles causes damage.
One major study suggested there was little significant damage done, that it was a harmless habit.
However, another study found that knuckle popping wore down the fluid-filled cushion area between bones, causing more and more friction with time and leading to arthritis.
In other words, your mother’s warnings about getting big knuckles may be correct.
Note that the popping of hip or ankle bones when you stand up from a sitting position is caused by something else.
That popping noise is a ligament that is passing over the joint, repositioning itself so the bones can move to a different position, sort of like a snapping rubber band.