query: Can cats control their purr?
passage: Purr
One hypothesis, backed by electromyographic studies, is that cats produce the purring noise by using the vocal folds or the muscles of the larynx to alternately dilate and constrict the glottis rapidly, causing air vibrations during inhalation and exhalation.[2] Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.[3]
passage: Cat communication
Cats often use their tails to communicate. A cat holding its tail vertically generally indicates positive emotions such as happiness or confidence; the vertical tail is often used as a friendly greeting toward human beings or other cats (usually close relatives). A half-raised tail can indicate less pleasure, and unhappiness is indicated with a tail held low. In addition, a cat's tail may swing from side to side. If this motion is slow and "lazy", it generally indicates that the cat is in a relaxed state. Cats will twitch the tip of their tail when hunting or when otherwise alert or playful. Abrupt, full-tail twitching indicates a state of indecision. A stalking domestic cat will typically hold its tail low to the ground while in a crouch, and twitch it quickly from side to side. This tail behavior is also seen when a cat has become "irritated" and is nearing the point of lashing out, typically by biting or swatting with claws extended. A cat may also twitch its tail when playing.