During physical adjustments, chiropractors stretch real quickly the joint they are trying to unblock. This quickly decreases the pressure (tension) in the joint, which in turn, creates a small gas bubble that bursts to become liquid again. We often compare this phenomenon with the opening of a soda can: depressurization creates a noise. It is by no means a bone that cracks. Also, when done by a duly trained chiropractor, adjustments are completely without risk. We recommend that you do not try to reproduce the movements of chiropractors at home as they’ve spent 6 years learning how to do it safely.