Harold's mama wanted me to make mention to the world about our 21 month old. Give the boy a picture of an evening sky and he will be able to identify the moon in the darkened sky. Point to it and ask, What is that? He will answer, "Moo", or "Muh", something to that effect.
Go out in the early evening or early morning that's sporting a waxing or waning crescent, point to it and ask, What's that? You will get the same response.