Many people crack their joints on a regular basis, whether out of habit or to relieve stiffness, however, until recently the mechanism that produced this sound has been poorly understood. Historically a number of causes have been proposed including recoil of ligaments or a sudden collapse of gas bubbles within the joint. Most recently a mechanism known as tribonucleation has been proposed which suggests that the rapid formation of gas bubbles within the joint causes the sound. Tribonucleation occurs when two closely opposed surfaces, separated by a thin viscous fluid, are distracted. The tension between the surfaces resist their separation until a critical point where they separate rapidly, creating a cavity (or bubble) within the fluid, much like a solid that has been fractured.
A recent study by Kawchuck et al. (2015) aimed to determine the exact cause of the noise by using real-time MRI on a cracking joint. The Metacarpophalangeal joints (the large joints in your hand at the base of your fingers) were used in the study by inserting the finger into a flexible tube with a length of cable attached that provided traction.
Photo taken from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0119470
A real-time MRI was then taken as traction was applied to the joint and continued until the cracking event occurred. The results of the study provide evidence that the FORMATION of gas bubbles within the joint causes the sound as opposed to the collapsing of a pre-existing bubble.
But the question still remains, is this cracking bad for your joints? On one hand the energy produced during cracking is enough to cause damage to the joint surfaces, however, research also shows that habitual joint cracking does not appear to show any long term harm. One interesting finding from the study was the presence of a flash of white light on the MRI, just prior to the joint cracking. Kawchuck et al state that further research in this area should help us understand the possible therapeutic benefits, or possible harms, from cracking your joints.