Why Do Cats Knead?

Kneading is “an instinctive trait in cats, who often knead on a soft surface, such as a blanket, other cats, or your lap. It appears to be relaxing and soothing—many cats will purr contentedly, drift off into sleep, or simply zone out and enjoy the motion.”
The act of cat kneading itself looks a lot like a human kneading dough. Cats repetitively dig their paws and pull them back up; they may also accompany their kneading with purring as well.