Estonian researcher Jaak Panksepp experiments
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Estonian researcher Jaak Panksepp did experiments on rats. When Panksepp tickled his rats on the neck, they made a chirping（唧唧声）sound, which the scientist believed was laughter. The rats liked the tickling so much, he reported, that they sought more of it. According to Scientific American, Panksepp's rats would follow his finger around to make him tickle them again.
While studies have shown that dogs and rats “laugh" when they play, it doesn't mean that all animals really laugh even if they make a laughter-like sound.
Take hyenas（土狼）for example. The hyena is well known for making a noise that sounds like a person laughing loudly. But studies show the hyenas don't make the noise because they think something is funny.
But make no mistake, when your dog welcomes you home, it's definitely laughing.
A. You can make your dog laugh at times
B. Laughter can relieve their nervousness
C. He found that they had a sense of humor and fun
D. The sound is a “forced, breathy exhalation（呼气）”
E. We've all wondered whether our dogs are smiling at us
F. Some would seek out toys and others would run toward the source
G. The “laughing" is actually a noise they make when they are excited or frightened
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