Cats spray to send a message – whether they are stressed or ready to mate. It’s important to know they don't do it out of spite. Spraying – or marking – is all about communicating through scent and it differs from urinating because when cats pee, they squat. On the other hand, when cats spray, they usually mark vertical surfaces. They stand with their backs to the object, twitching their tails and spraying urine, usually at another cat's nose level. In this case, the urine may form a thin line instead of the paddle formed when the cat has peed outside the litter box.