James Geary in The Atlantic:
Everything is formed by habit. The crow’s feet that come from squinting or laughter, the crease in a treasured and oft-opened letter, the ruts worn in a path frequently traveled—all are created by repeatedly performing the same action. Even neurons are formed by habit. When continuously exposed to a fixed stimulus, neurons become steadily less sensitive to that stimulus—until they eventually stop responding to it altogether. Anything that’s habitually encountered—the landscape of a daily commute, storefronts passed on a walk to the bus stop, photographs arranged on a mantelpiece—tends toward invisibility. The more we see a thing, the less we see it. Familiarity breeds neglect. Once perception settles into a comfortable pattern, we fall asleep to it. Only when the pattern is broken do we notice there is a pattern at all. The chains of mental habit are too weak to be felt until they are too strong to be broken, to paraphrase Samuel Johnson. Wit, whether visual or verbal, can make the commonplace uncommon again by breaking the habits that render perception routine. We tend to define the quality of wit as merely being deft with a clever comeback. But true wit is richer, cannier, more riddling. And the best of it is often based on a biological phenomenon called supernormal stimuli.
…Supernormal stimuli are key to certain kinds of wit, too, deliberately skewing or exaggerating our usual patterns of perception. The great silent comic Buster Keaton is a case in point. In The High Sign (1921), as Keaton settles down on a bench to read his local daily, he unfolds the paper to standard broadsheet format. He soon notices, though, that the newspaper is bigger than he expected, so he continues unfolding it—first to roughly the surface area of an ample picnic blanket, then easily to the proportions of a king-size bedsheet, until he’s finally engulfed by a single gigantic swath of newsprint.