I guess one could think of the sternum as the "backbone" that holds the ribs together, keeps them in place. The sternum and ribs are a cage like system that protects your heart and lungs from injury....
because you touch it.
Your joints are releasing the synovial fluid and makes a cracking noise.
Almost immediately. If you paint each nail and watch for a few seconds you can actually see it crack.
So your body can release its junk, which is part of the human digestive system. Also known as 'pooping'.