Dr. Eric Walle, a psychology professor at the University of California Merced, accidentally discovered early in his career that babies who are walking have significantly larger vocabularies than children who are crawling. After he replicated the finding across languages, he set about using LENA technology to try to determine why.

“It wasn’t just a matter that these kids were older. It was that these kids had begun walking,” he said. “Walking seemed to give them an added boost to their vocabulary development.”

His process of discovery is highlighted in episode six of a new Netflix documentary series, Babies. The episode follows a baby named Penelope and her mom during a LENA Day, while Dr. Walle explains how he examines the data provided by LENA.