She began by getting six adolescent rats accustomed to a 300-square-foot room fitted with boxes and barriers behind which they (or Reinhold) could hide. She also habituated the animals to her by stroking them, chasing them with her hands, and tickling them.

After one month, Reinhold taught them to seek. She placed them in an open box and walked to a different part of the room. If they approached her, she rewarded them with tickles. At first, she stood in the open, but once the rats got the hang of approaching her, she incrementally raised the difficulty by partially hiding, then hiding fully while the rodents were watching, and finally hiding while the box was closed. Similarly, to teach them to hide, she rewarded them for leaving their box, then for scurrying to another area, then for finding and staying in a hiding spot. Crucially, she never remunerated the rats with classic rewards such as water or food. She treated them only to tickles and social contact.

All six rats learned to seek, and five also learned to hide. They clearly understood the rules of the game, and played strategically. When seeking, they searched systematically, beginning with past hiding locations. When hiding, they chose opaque boxes instead of transparent ones and kept quieter. They also seamlessly switched between the two roles, taking their cue from whether the starting box was closed (indicating “seek”) or open (indicating “hide”).

The rats learned the game in only a couple of weeks, which is “impressive in neuroscience,” says Juan Ignacio Sanguinetti-Scheck, who also took part in the study. “Animals can take months to learn tasks, even monkeys, but we’re generally trying to teach them to use joysticks or things they’d never do in normal life.” Hide-and-seek, by contrast, draws on behaviors such as concealing, finding peers, and switching roles, which aren’t just natural parts of rat life, but also frequent parts of rat play. In retrospect, it was the perfect game for uniting two different species. “It’s a clever and innovative approach,” says Gordon Burghardt of the University of Tennessee. “Many animals play with other species and engage in peekaboo, tug of war, or tag, but this example does seem unexpectedly complex.”

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Why did the rats play along? It’s possible that they were going after tickles and other social rewards. After all, for two decades researchers have known that rats enjoy being tickled and react by producing ultrasonic chirps that can be compared to laughter. But Reinhold found that once discovered, the rats would often run away and re-hide, delaying their reward and prolonging the game itself. “It seemed really playful,” she says.

She and her colleagues believe that rather than pursuing rewards, the rats were playing for the sake of it. They played because they had fun. For a start, and this is an unusual but welcome line to see in a scientific paper, “the animals looked like they are having fun,” the team writes. When they reunited with Reinhold, they frantically jumped on the spot—a behavior delightfully known as freudensprung, or “joy jumps.” They also teased Reinhold by repeatedly getting close and running away.