They told us that we will get arthritis, and that our hands will be trembling to the end of our life, and one doctor decided to prove it wrong.
The fluid in the joints is called synovial liquid and due to a drastic drop in pressure, when the joint is stretched, half of it turns into gas. The sound that we hear is actually the bubbles of gas coming out of the liquid.
To refute the thesis about arthritis, Dr. Donald Unger, for 60 years he was cracking his joints only on the left arm, and never on the right. Arthritis did not appear on the left hand, nor on the other. For his research, he won the Nobel Prize for medicine.