Comment by s_m_t

That doesn't seem too bad to me. It isn't trivial to prove the earth is flat. 10th percentile IQ seems to be around the functional illiteracy threshold. The military won't take them because as much as they have tried they can't seem to get that 10th percentile trained in just about anything. If such a person looks out into the distance, sees that things look flat and not like a sphere, and then watches some video with half-baked reasoning telling them they are right then I'm not sure how you could realistically convince them otherwise.