Each movement is categorized in Action Unit (AUs). When you puff your cheeks, it's known as AU13. The Frontalis muscle, located on the forehead, is responsible for AU1 or the "Inner Brow Raiser". Over the course of their extensive research, Ekman and Friesen determined that there are at least 19 different versions of smiles! For more information and additional resources on FACS, visit the Data Face website.

If you live in the Bay Area, you can see a special exhibit at San Francisco's Exploratorium with more of Dr. Ekman's photos. It's open through May 11.

Watch the "Emotions Revealed" TV Story online, as well as find additional links and resources.

Jenny Oh is an Associate Producer for QUEST on KQED Television.