Knuckle cracking. What is happening when you do it and what is the sound from?
This video shows you what is happening inside your knuckle joint as it is being cracked by using the ultrasound machine.

For many years it was thought that cracking your joints would lead to arthritis. However this is an old wives' tale, there has been no evidence to prove this hypothesis.
Donald L. Unger was a Californian medic who only cracked his knuckles on one hand and not the other… for 60 years! This was to see if the theory was true and if it worsened arthritis, however at the end of the 60 years, he didn’t have it any worse in one than the other.

But what is actually happening in the joint when we crack them?
A group of researchers from the University of Alberta thought the same thing. In April 2016 they released a paper about how they used MRI scans to image the finger joins when they were being cracked. Their conclusion was that the sound came from the formation of air bubbles in the synovial fluid (fluid around the joint).
The fluid contains the gases oxygen, nitrogen, and carbon dioxide. When you pop or crack a joint, you stretch the joint capsule and gas is rapidly released, which forms bubbles.
Later, near the end of 2015, the theory was tested further by Californian scientists. They used an ultrasound machine however, as it produces images much quicker and in real-time.
They recruited 40 participants, 30 of which were regular knuckle crackers, 10 were not.
They cracked the metacarpophalangeal joint (the knuckle) and observed a “bright flash of light exploding” in all 400 joints.
The researchers suspected this to be the change in pressure in the synovial fluid.
It still isn’t 100% clear as to what causes the popping sound but the research suggests it is from the formation of the bubbles, as they “consistently saw the bright 'flash' in the joint only after they heard the audible crack. Never the other way around.” which supports the bubble formation theory, and no the bubble popping theory.