Scratch a friendly cat behind the ears and she’s likely to reward you with a deep purring. Have you ever wondered exactly how a cat makes this noise, and why a cat would want to do so in the first place? There have been a number of theories about how cats purr. For example, scientists once thought that purring resulted from the turbulent flow of blood within the cat’s chest, rather like the babbling of water over an uneven river bed. Another theory holds that purring is essentially a type of snoring, where the cat’s air stream is interrupted by a structure in the cat’s larynx. The most likely explanation, however, is that purring is caused by a rapid, rhythmic contraction of the muscles of the larynx and diaphragm.