One knee cracks like a huge knuckle. Why?

Long time ago I remember asking the same question about why the bones pop, like when we pop our fingers. One answer I got was that when we pop our fingers is not the bone that pops but the air that its in between the joints. If this is correct, maybe you retain or capture air in your knee and when you start moving the joints get together and pop the air out of the joints making the popping sound. I am almost sure that this information was also given by a doctor. Take note of the kind of movement you do when the knee pops. Maybe is when your leg is strait or when is bent. Hope helps! Maybe is normal. You know how different people can do different movements with thier body parts like the elastic person that can do many tricky movements that not everyone is capable to do.