Why do cats purr?

A purr is produced when signals from the brain cause the muscles of the larynx to twitch 25–150 times per second, opening and closing the space between the vocal cords so that air vibrates on both the inhale and exhale. While purring most often signals contentment, cats also purr when injured, frightened, or giving birth. Intriguingly, the frequency range of purring overlaps with vibrations shown to promote bone density and tissue repair, leading some scientists to suggest purring may be a self-soothing and even self-healing mechanism.