Every cat owner loves to hear their pet purring beside them, like a contented little Roomba. But the idea that cats purr because they’re happy or satisfied may not be entirely accurate. Yes, they purr when they’re being petted or nursing their kittens, but they also purr in stressful situations—which has led to an ongoing debate about why they do it and what purpose it serves. Researchers have determined that cats purr at frequencies between 25 and 150 Hertz, a frequency range that “can improve bone density and promote healing.”

According to Leslie Lyons, an assistant professor at the School of Veterinary Medicine at the University of California, Davis, cats may purr as a way to stimulate their bones and muscles without expending any real energy or stressing their bodies. “Although it is tempting to state that cats purr because they are happy, it is more plausible that cat purring is a means of communication and a potential source of self-healing,” Lyons told Scientific American.