A microexpression is a brief, involuntary facial expression shown on the face of humans according to emotions experienced. They usually occur in high-stakes situations, where people have something to lose or gain.
Unlike regular facial expressions, it is difficult to fake microexpressions. However, no one can fake above a certain extent. Despite claims of effectiveness, many in the field of psychology regard using microexpressions with great skepticism since no peer-reviewed journal articles have been published on their use in lie-detection.
Microexpressions express the seven universal emotions: disgust, anger, fear, sadness, happiness, surprise, and contempt. They can occur as fast as 1/25 of a second.