Imagine you are the survivor of a horrible car crash. One day, while you’re walking down the street, you hear a car horn followed by a screeching noise. Before you get a chance to look around and figure out what happened, you feel a sudden rush of adrenaline. Fear paralyzes you from head to toe, and your mind fills up with images of the accident in which you were involved not long ago. It may look like you’re overreacting from the outside, but from the inside, everything feels so ‘real’ and overwhelming. And so, you sit there shaking and waiting for something horrible to happen.