Why do cats knead blankets?

You're sitting on the couch when your cat hops up, settles onto your lap, and starts doing something extremely weird. Push-push-push with the front paws, like a tiny baker working invisible dough. Sometimes the claws come out. Sometimes there's purring. Always there's that focused, blissed-out look on their face. What is happening?
Here's the thing: your cat thinks they're a kitten again. When cats are just a few days old—blind, helpless, barely bigger than a mouse—they knead their mother's belly while nursing. The pushing motion helps milk flow from her mammary glands. Push-push-push means "more milk, please." It's one of the first things a kitten ever learns to do.
