One of the great and endearing mysteries about cats is their use of purring to show affection.
But cats also purr when in danger or while giving birth or dying.
Feral cats will even purr during a standoff with another cat.
Cats only purr in the presence of humans or other cats.
Because they are born blind and deaf, kittens depend on feeling the purring of their mothers to find comfort and a place to nurse.
The kittens themselves start purring at one week.
The purring of all adult cats derives from this mother-kitten experience, a form of communication often accompanied by kneading the paws as they did while nursing.
Purring is by choice and is exclusive to domestic cats in that it occurs uninterrupted both during inhaling and exhaling.
Big cats make a similar noise, but only while exhaling.
Raccoons also produce a purring sound, but again only while exhaling.
Cats choose to purr, but how is another question still a mystery to science.