Named after 19th-century French neurologist Guillaume Duchenne de Boulogne, the Duchenne smile activates two key muscle groups. The first group is associated with the corners of the mouth ...

The first group is associated with the corners of the mouth—where, for example, the risorius (from the Latin to smile) draws the corners outward and the zygomaticus major muscle lifts them.

This alters muscle tensions around the mouth, creating forces that further narrow the face and jaw. The constant open-mouth posture also changes how teeth erupt and align.

All of these things help build up the jaw and mouth muscles. As most toddlers move closer to age 4, their skills should improve on their own.

The specific mouth muscle you’re engaging when sipping a straw is called the orbicularis oris, according to board-certified dermatologist Dr. Blair Murphy Rose.

At a certain point, she struggled to lift the corners of her mouth. So Ms. Kawano, then 43, decided to learn how facial muscles work.