Numbers are complicated

Zeke's bedtime routine has come to include the song, "Green Grow the Rushes, O," which is interesting. He used to love the as a baby, and then we sort of forgot about it, maybe because it's so long, and it never seemed like a great bedtime song. It's a traditional cumulative counting song, which begins with one and runs through twelve. Anyhow, he now wants to hear it before bed. Tonight, presumably wanting to postpone the inevitable, he interrupted me halfway through the song and instructed me that I had to sing all the way through twenty tonight. I laughed and told him I didn't know any more after twelve, but that I'd see what I could do. He answered, "If you don't know the other numbers, I can teach you." I said O.K. After I got to twelve, and stopped, we had this exchange:

Z.: Daddy, you forgot two numbers. Thirteen and fourteen.
Me: I don't know those.
Z.: I can sing them for you.
Me: Okay.
Z.: (after a pause, in a speaking voice) Thirteen. (pause) Fourteen.
Z.: (after another long pause) Daddy, thirteen and fourteen are hard to explain. Very hard to explain.