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Comic strip of a ghost and a person with the American flag pasted on the head. The ghost repeats "Boo!" in the first three panels without getting any reaction, but when it in the fourth panel says "kg, cm, km, °C" the American gets scared and screams "AHHHH!!!".
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The actual reason the us hasn't switched is the many billions of dollars it would cost for basically no tangible benefit. There are probably better uses of that money if we actually got to spend it on what we wanted, like social programs.