When we view something, we notice big details—the people, the forest they are in, perhaps the house in the back—and fail to zero in on less important features like the number of shrubs in the forest or the house's finer details (hint, hint). Dan Simons, a psychologist at the University of Illinois at Urbana-Champaign, says that's because if we spotted everything, we'd be unable to focus our attention. So our brain fails to log details it deems unimportant. When we flip back and forth trying to find them, we can't because we never noticed them in the first place. However, once we do see the disparity, it gets stored as one of the obvious elements, and then we can't seem to unsee it.