Moses

This is from our math book, an illustration of a geometric pattern. The simplistic description of the pattern was: "Take away bottom row of triangles". That's not very mathematical, so I wondered aloud if I could find an equation to represent the formula. "Let's see," I said, "You start with 15 triangles, then you have 10, and finally you have 6.'

Moses, waving his arm wildly, interjects, "That's not 6 triangles, there, Mrs. Matters. There's 9 triangles."

"Nine, Moses? I count only 6."

"You didn't count the white ones, Mrs. Matters."