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. You can feel a cat's purring in both the throat and stomach, which is where these muscles are located.
Exactly why a cat purrs is also a matter of scientific debate. Purring clearly serves a social function, as a type of communication, because cats never seem to purr when they are alone. However, exactly what is being communicated is unclear. It may be a signal of submission. Cats often purr when they are around a dominant, aggressive rival. It might also be a signal of affection, because mother cats purr to their kittens, and kittens purr back to their mothers. Cats also purr in anticipation, or supplication, such as when they're about to be fed.
In general, purring seems to be a good thing, a sign of a happy cat.