Why does my cat paddy-paw? easy ten points?

I call it "squidging". It's commonly thought to be a relic from their time as kittens - kittens do this around their mother's teats to stimulate milk flow when they are drinking - this is also why they often salivate whilst doing it. Some people say they only do it if they were taken from their mother at too young an age, but I haven't met a single cat yet who doesn't do it, so I don't know how true that is.