because you touch it.
Your joints are releasing the synovial fluid and makes a cracking noise.
Well actully, when you crack your back, its not like a bone or joint cracking. Its sort-of "bubbles" in your back from spinal fluids.
So your body can release its junk, which is part of the human digestive system. Also known as 'pooping'.
Almost immediately. If you paint each nail and watch for a few seconds you can actually see it crack.
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Ratchet