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The second principle of memory concerns emotions. If something made us incredibly frightened, elated, sad, or angry--four of the primary human emotions--we're more likely to remember it. This is because the brain creates neurochemical tags, or markers, that accompany the experience and cause it to become labeled as important. It's as though the brain took a yellow fluorescent highlighter to the text of our day, and selectively marked up the important parts of the day's experiences. This makes evolutionary sense--the emotionally important events are probably the ones that we need to remember in order to survive, things like the growl of a predator, the location of a new freshwater spring, the smell of rancid food, the friend who broke a promise.