My basic assumption is that people aren't stupid but they *can*
  get locked into a mode of thinking. This can affect anybody,
  from a child that refuses to eat french fries because they were
  scared by a clown while eating french fries once, or an
  intellectual well versed in a variety of world issues.

  I've worked with people of all ages and "intelligence levels"
  and while I have seen thick-headedness (even in myself), I
  haven't found genuine intelligence differences between people.

  Language barriers? Certainly. Comprehension difficulties?
  Definitely.

  But I've found little that can't be explained to anybody given
  the right metaphors/analogies/explanations.