Yesterday morning I put a baseball cap on him backwards and said “Hey dude!” – shaking my head to the side and snapping it back. He thought this was very funny and laughed every time I did it (like in this recording, noted this entry – sheesh, listening to that again I was almost torturing him and not tickling). He picked up the word right away and started doing the same thing, saying “Dude!” clearly with a jerk of his head to the side, waiting for me to either do it in turn or laugh.
Tia said he was doing it again later that day, but when she did it back the novelty had worn for him (as with other new amusements).