You're entering a train car, a restaurant, a local store. As you step inside, you scan the people there. You don't know any of them, yet in seconds you register impressions of them all. He looks friendly, she appears evasive, that teenager seems threatening. Even as you're assessing the factual cues of their bodies--gender, skin color, height, age--you already seem to know whom you perceive as likable and whom you should avoid.
The fact that our brains can reach such rapid conclusions is astounding. It is also troubling: despite the paltry information available to the brain in those initial moments, our first impressions can color our continued perception of an individual, regardless of whether his or her later words and actions contradict our hasty preliminary view.