Rats just want to have fun. This vocalization came on right away, and more intense than before. And the data have literally been flowing ever since. It's quite easy. You get the most laughter at the nape of the neck, where they direct each other's play behavior.
Rats register their gratitude with little nips.
The amazing thing is, prior to starting this line of research, I had never been bitten by a rat. But since I've started this, I've been bitten hundreds, if not thousands, of times, but always in a playful way. The skin has never been broken. Panksepp knows people may laugh at his research, but he doesn't care since his goal is to track the biological origins of joy. On the playground, where kids are running and playing.