Why do cats hiss?

A hiss is made by forcing air rapidly through the mouth with the tongue arched, and it is usually paired with flattened ears, an arched back and puffed-up fur. It is a defensive signal rather than an attack: the cat is saying it would prefer not to fight. Some researchers have suggested cats may mimic the hiss of snakes, which many animals instinctively fear, though this is speculative. Kittens can hiss within a few weeks of birth. Cats commonly hiss at unfamiliar animals, at vets or when in pain, so sudden hissing in a normally calm cat can be a sign of illness or injury.