Neuroscientist Jaak Panksepp first heard rat "laughter" when he observed rats at play. For a study published in the journal Behavioral Brain Research in 2000, he and his team tried to elicit the same laughing sounds by tickling the rats with their hands. They found that the rats seemed to enjoy and seek out the tickling treatment. Rat laughter sounds at about 50 kilohertz, so scientists need special machinery to hear the high-pitched chirps. According to Panksepp, the best way to tickle a rat is to start at the backs of their necks, then flip them over and tickle their bellies.

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Laughter isn't the only thing you get when you tickle a rat. Tickling can also affect their behavior overall. A 2012 study published in the journal PLOS One found that tickled rats were also more optimistic: they were more likely than un-tickled rats to press an unknown lever to see what would happen. A 2015 study published in the journal Applied Animal Behaviour Science found that tickling a rat right before giving it an injection can put it in a better mood, making it less likely to struggle and complain. That's important, because stressed out animals can change the results of all sorts of scientific experiments. Happy rats means happy science.

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