The building blocks of FACS are 46 action units  (AUs) such as a wrinkling of the nose, a puffing of the cheeks, dilation of the nostrils, squinting, and so on.
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There are forty-six action units, like squinting and wrinkling the nose, that comprise the basic elements of FACS.
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The action units of FACS involve only body movements, not facial expressions like squinting.