From @kellyfosterkittens: “Goooood morning! ❤️ A toothbrush feels like a mother cat’s tongue, providing comfort and cleaning them! Can you tell by the purrs that Luna is enjoying this? 🥰” #catsofinstagrampic.twitter.com/VVUjuIC5FV
This tiny kitten thinks the toothbrush is her mother's tongue so she enjoys being "licked" by her mother. She starts to purr and offers up her chin for a wash. I think it is the first time that I've heard such a young kitten purring and you can see her left arm juddering slightly in pleasure. That is my reading of her behaviour. It is a form of feline allogrooming only the human is using a toothbrush very gently to simulate an adult cat's tongue.