Why do we get a sound when we crack our knuckles?

I've been wondering about this a long time and someone had told me the answer but I had forgotten, so today I decided to look it up!!

Basically, because we stretch our fingers, our joints get seperated and the liquid between the joints gets stretched too. And as the liquid gets stretched, which is equivalent of reducing pressure, (think of a water filled syringe which is sealed at the tip, and pulling the plunger out), the air dissolved within the water starts forming bubbles and pops out of the surface. This is what makes the noise.

Additionally, it takes about half an hour for the air to redissolve back into the fluid, and that's why it takes some time before you can crack your knuckles again!

And finally, it's not considered harmful except for making your grip a tad weaker! So, don't do this if you're into arm wrestling!

The detailed answer is here.