Scientists know that rats like to have their bellies tickled, so they used that as basis for testing happiness in rats. They found out that the ears of rats undergoing tickling became droopier and pinker - subtle signs of being relaxed and happy.

Scientists know that rats like to have their bellies tickled, so they used that as basis for testing happiness in rats. They found out that the ears of rats undergoing tickling became droopier and pinker - subtle signs of being relaxed and happy.