Why do your bones crack?

Why can some people crack heaps of bones and some people can't?

• Okay so here the thing, I can crack my neck, all joints in my fingers, two joints in my toes, jaw, knee, somewhere near my pelvis, shoulder blades, elbows, the back of my neck, wrist, ankles basically anything but my back. Anyway I get this from my dad, but my friends, my mum and basically everyone else I know can't do this why not?? Would it help if I saw I'm lactose intolerant and don't eat enough meat or vegetables? But how come some peoples bones can crack and some peoples can't? And what makes them crack isn't it like an air bubble or something, well thats just what my dad told me anyway...So please someone help me out I am so curious on why this occurs...

• Answer:

  "Cracking" is caused by an air bubble that is produced when a joint is "distracted". Basically your joint has normally no air in it. It has a tiny amount of fluid. When pulled apart, the fluid is not able to expand, and a little vacuum develops which gets quickly filled with gas.. Imaging a sucker arrow/dart on a glass surface...to make the arrow stick, the sucker has to be slightly wet. When the arrow is pulled off from the glass surface you hear a "pop". This is the same as what is happening in your joint. To do this consistently, the joint has to be supple, and the amount of fluid has to be right. People whose joints are not so supple or have too much fluid will not be able to crack their joints.