There are a number of different things that your mother may have told you that just don’t pan out.  The biggest one I’ve heard is that cracking your knuckles causes arthritis.  Of course, ironically enough, I now have arthritis, and I had a big habit of cracking my knuckles as a kid, so of course I would.  But realistically speaking, it doesn’t work that way.  When you crack your knuckles, the sound is just air being released from between those joints, and frankly harm is really never done. 

 Wear and tear happens over a lifespan for most individuals, and sometimes your body actually ends up being the one to attack your joints and otherwise cause arthritis.  However, cracking your knuckles to release that air buildup that sometimes happens and the nitrogen in your joints will not actually cause arthritis.  It may stretch that area out a bit, but the sound is completely harmless, just a general rush of gas. 

There is no scientific study to suggest that it would do anything else or do any kind of harm at all or for that matter make your knuckles bigger, which was another one I heard from my mom when I was a kid, mostly because my sister was always popping hers and we copied her.  I think it mostly just annoyed my mom.