You're driving down the bustling main street of a picturesque little town you have never visited before. The traffic light turns red, you stop, and an old lady steps into the crosswalk from the left. All of a sudden you are overcome with a feeling that you have been here before--in the same car, at the same crosswalk, with the same woman stepping off the curb in the same way. Yet by the time she reaches your front bumper, you realize the scene no longer matches quite so well with what you thought you were recalling. And you do know you have not been here previously. The familiarity is broken.
Various studies indicate that 50 to 90 percent of us can recall having had at least one such déjá vu incident in our lives. We experience a vague sense of having encountered a situation before, identical in every detail, even though we cant say when the first event took place. Usually the sensation lasts only a few seconds. Teens and young adults stumble on the dreamlike state more often than older adults, yet people of all ages experience déjá vu, especially when they are either fatigued or overly aware because of stress. A few people sense the inverse of déjá vu, called jamais vu. When they encounter a familiar person or place, they nonetheless insist they have never seen the individual or scene before.