Apparently, if you tickle a rat it will respond with vocalizations that scientists have good reason to interpret as happy ones. Basically, it's the rat equivalent of laughter, only at ultrasonic frequencies that the human ear can't detect on its own. What's more, tickling rats on a regular basis appears to reduce the negative effects of stress in their lives. Scicurious' write up of this research includes the amazing quote: "For the “tickling treatment”, rats were tickled once daily, in two sessions of two minutes each, for two weeks." Also, there is video of this.