Cracking your knuckles does not actually hurt your bones or cause arthritis. The sound you hear is just gas bubbles bursting.

So what is actually happening then ? Well when you crack your knuckles you move your joints in and out of position this causes pressure on the Synovial fluid (a lubricant surrounding your joints) when this happens bubbles form and burst causing the cracking sound you hear. As for this causing other problems a study of 300 people who cracking their knuckles on a regular basis was performed and no evidence was found it causes Arthritis