 From an early age many of us are taught that when cats purr, they're telling you they're happy. While purring is thought to be partly voluntary and partly instinctive, research suggests that cats can purr for various reasons, using the soft rumble as a way of communicating and as a form of self-soothing or even healing. This is why cats will often purr when they're injured, or after a stressful event. A 2001 study published in the Journal of the Acoustical Society of America showed that domestic cats and some species of big cats, including pumas and cheetahs, could purr at frequencies optimal for pain relief and even bone repair.