Wondering where your teen’s mind is? A study in the Proceedings of the National Academy of Sciences suggests adolescents may be quite busy adding brain cells during puberty, and it might just have to do with sex.

“In school we were always taught the cells you’re born with are pretty much all you get,” said Margaret Mohr, a doctoral candidate in neuroscience at Michigan State University and one of the study’s authors. “We’re finding that’s not true at all.”

Adults can add neurons in the hippocampus, an area associated with learning and memory. But enough about your hippocampus. Let’s talk about your teen’s amygdala, which processes memory and emotional reaction.

Your amygdala might pick up on someone’s sexual interest, perhaps. Or read that someone is spoiling for a chance to prove his dominance. All of that is useful around the time children are about to transition into the tricky social interactions of adulthood.

Researchers don’t use real teens, they used the next best thing: a pubescent male hamster. They injected the rodents with a commonly used substance that helps mark the birth date of cells, then put them in different environments – an “enriched” one with a wheel to run on, another without much to stimulate them.

Then researchers gave all the male hamsters, now a few weeks older, a shot at a sexually available female. Then they took a close look at the male hamsters’ brain cells. Sure enough, those cells born during puberty showed signs of activity.

The fact that cells known to have been born in puberty were active during these interactions suggest the cells had been incorporated into the circuits that drive male sexual behavior.

Having a wheel to run around on made a difference, too. Those hamsters were more likely to retain these teen cells into adulthood, the study found.

“In humans, exercise and social interactions are important for your well-being,” Mohr said. “We know exercise helps these new cells survive.”