Mark is worried that Arthur may be smarter than the two of us combined.
He's discovered homophones, specifically, that dam is a homophone of damn.
Scene: Sunday evening dinner.
Arthur (squirming in his high chair): "Damn." (Mark and John's heads swivel as if they were searchlights transfixing a convict) "Arthur make a dam. In the park." (Mark and John go back to pretending to eat dinner.) "Say 'blast' instead. Damn... Arthur make one in the park."
Oh well... at least a half hour later he said "This video is really boring. Watch Gordon [one of the Thomas the Tank Engine trains] instead."
So we know that his language centers are working.
I did sit down with him and tried to explain that when he said "damn" it makes Mark sad. This seemed to make an impression on him, and we'll see if it is a lasting one.