Fashions change but what can a person wear that is never out of style?

Why this works

This riddle plays on the concept of "fashion" and "style," typically associated with clothing trends that come and go. However, a smile is an expression that transcends time and trends—it’s universally appreciated and always appropriate, making it a timeless accessory. The clever twist is that while we often think of garments when discussing fashion, the answer shifts the focus to an emotional expression that enriches both the wearer and those around them.