The men who tickle rats
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.