Just as, in movies, intelligent people are always super-genius polymaths, never just someone who's above-average in their chosen field. Or someone with a high libido is portrayed as a sexually-driven animal, rather than just a normal person who likes to get laid every night. Or if you're the adventurous sort, you must be an Evel Knievel wannabe, unable to turn down any dare at all, rather than just someone who might like to ride the Stratosphere (that roller coaster atop a tall tower in Vegas).
The deaf people in movies, of course, are that way because how much fun is it to watch a movie where one of the characters is constantly cupping his ear and saying, "What?"
In movies, everyone's traits are exaggerated to a ridiculous degree. It makes it easier to tell your entire story in two hours to a roomful of popcorn-munching strangers. TV shows can have subtler characterizations (Abby on NCIS, for instance, or Dr. Brennan on Bones), because they have more time to tell their story arcs. (A given story must finish in 30-60 minutes, including commercials, of course - but the characterization can be spread across many such stories.)
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.