We get this question all the time in our office.  The fear is that the sound is actually coming from the bone or the cartilage in the joint.  We always answer by letting people know that it is actually tiny gas bubbles forming in the joint.  We know that the adjustment helps to restore normal mobility in the joint, increase blood flow to the area which delivers oxygen, and causes your body to release endorphins (our natural feel good brain chemical).  Some really cool things happen when we adjust you and now you can actually see it on MRI.   Scientists at the University of Alberta wrote an article and released a video MRI of the bubble forming in the joint capsule which causes the "pop."  Check out it out HERE.