A small muscle movement is all it takes to turn up the sides of your mouth, but a smile packs a surprising amount of power! Studies show that people think they already know someone if that person smiles at them, even if it's an absolute stranger. The theory: Scientists believe the need to bond is rooted in our evolutionary past. Survival was more likely when people combined forces, so humans acquired the smile as a way to signal friendliness and to induce an agreeable sense of shared history, whether there was one or not. Today, when someone smiles, a glum mood is lifted, an apology is accepted, a person's shaky self-confidence gets a boost, a deal is struck, a physical attraction is communicated. But change the cast of a smile and the consequences shift. A rival grins to get under your skin; a bully smirks to unsettle his mark. Understanding the nuances helps ensure that you send—and receive—the right mouth message. Here, some intriguing insights about this familiar gesture...