Some people love it, some hate it. Sometimes it happens spontaneously, and sometimes people deliberately crack themselves all the time. Is it safe? Is it helpful?
Almost all joints contain synovial fluid that is enclosed in a fibrous ligament capsule joining the 2 bones. If a joint is moved so that the bones are distracted apart (like pulling a knuckle), or stretched in a specific way (often quickly), pressure reduction in the joint can cause gas that's normally dissolved in the fluid to form a bubble (like when pressure is released from a champagne bottle). We call this cavitation.
It is thought that either the formation or sudden collapse of the gas bubbles results in the popping noise we hear. It can take 15-20 mins for the gas to fully dissolve before the same joint can pop again.
Why do it?
There is no convincing evidence that bones are put back in place, but joint cavitation does often result in a temporary reduction in pain and muscle spasm, allowing increased movement.
Is it safe?
Studies on people that have cracked their knuckles for many years show no increased incidence of finger joint arthritis, however grip strength on average was reduced. Similarly we see lots of people that habitually crack their neck present with weak stabiliser muscles, leading to recurrent neck and head ache. There is also a remote but real risk of serious adverse neurovascular effects from neck cracking, and this should be left to health professionals.
Try to limit cracking your joints, especially repeatedly, and especially the neck.
Cracking from cavitation shouldn't hurt, or occur each time you move.
If you feel achy and tight, physios may include cracking manipulation with your consent as part of their treatment, but specific muscle retraining will also be part of the solution.