I talk with my hands because it helps me guide my attention and keep on track about what I want to say. Like I'm always, in a sense "seeing" what I'm talking about, so moving my hands around relative to what I'm seeing in my mind helps keep me focused. I also become very clumsy when I do this, so its not uncommon for me to walk into, trip, or knock something over when I really get talking (I'm great at parties!).
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"To the end, my dear." ~ Stravinsky