Microexpressions occur:

Microexpressions are very brief facial expressions people make when feeling certain emotions. They occur within a fraction of a second and are involuntary. Here’s how they can help you deliver better customer service: → Facial expressions reflect a person's true emotions; → You appear more confident in sales negotiations; → You can better understand your customers and build rapport; → You can better control the conversation; → You can gather feedback your customers might not even know they are giving.

In the video below, you can see Paul Ekman, an American psychologist and expert in the study of emotions and facial expressions, explain microexpressions. https://www.youtube.com/watch?v=fhv8Ftaq1LI

Source: "Micro Expressions" on paulekman.com Photo by Andrea Piacquadio from Pexels