Why Danica McKellar Accidentally Smiled During Her ‘Wonder Years’ First Kiss (VIDEO)

Danica McKellar is still probably best known for a role she played almost 30 years ago, when she was just 12: Winnie Cooper on the ’60s-era coming-of-age sitcom The Wonder Years. But she’s also a mathematician who has authored books that encourage teenage girls to continue their interests in the STEM fields. Her first book, for instance, was called Math Doesn’t Suck: How to Survive Middle School Math without Losing Your Mind or Breaking a Nail.

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So she was a perfect subject for the PBS web series The Secret Life of Scientists & Engineers, from the producers of the network’s venerable science series NOVA. The second segment featuring McKellar debuts at midnight, and in our exclusive clip from that episode, she describes the famous first kiss she had with Fred Savage’s character Kevin Arnold in the show’s pilot.

Did you realize that McKellar smiled in the take that made it to air? Neither did we, and considering that Winnie was supposed to be sad over her brother’s death in Vietnam, it wasn’t supposed to happen. Watch McKellar describe why it happened; her excuse is pretty darn good, if you ask us.