I used to have a teacher who always gave me the line about smiling taking fewer muscles than frowning. I just told her I wasn't frowning, but she didn't believe me. :S
Most of the time people just think I'm really serious (they don't usually say "intense", but sometimes), but any sign of irritation makes them think I'm extremely angry. My mom once feared for her life when I approached her in a disapproving manner. I can only assume that she deserved whatever disapproval she got, but I do not remember what had happened. What I do remember is being pursued around the house by my dad when all I wanted was to be left alone. He thought I was afraid of him and couldn't understand that I just wanted him to GO AWAY.
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"If knowledge can create problems, it is not through ignorance that we can solve them." - Isaac Asimov