Pffft, grown up

Tonight Nolan was reading "Dance, Dora, Dance" to me and when he would get to words he didn't know, he'd say the word that he thought it resembled. When I would correct him on the bigger words, he'd repeat them and keep reading. But, when we got to one particularly long word, Nolan said "geez, do you know every word?!?" and I said, "yes". He proceeded to roll his eyes and say, "pffft, you really are a GROWN up." When did he catch on?!?