Inside a Berlin neuroscience lab one day last year, Subject 1 sat on a chair with their arms up and their bare toes pointed down. Hiding behind them, with full access to the soles of their feet, was Subject 2, waiting with fingers curled. At a moment of their choosing, Subject 2 was instructed to take the open shot: Tickle the hell out of their partner.

In order to capture the moment, a high-speed GoPro was pointed at Subject 1’s face and body. Another at their feet. A microphone hung nearby. As planned, Subject 1 couldn’t help but laugh. The fact that they couldn’t help it is what has drawn Michael Brecht, leader of the research group from Humboldt University, to the neuroscience of tickling and play. It’s funny, but it’s also deeply mysterious—and understudied. “It’s been a bit of a stepchild of scientific investigation,” Brecht says. After all, brain and behavior research typically skew toward gloom, topics like depression, pain, and fear. “But,” he says, “I think there are also more deep prejudices against play—it’s something for children.”