Paint by numbers is a system where a picture is divided into shapes, each marked with a number that corresponds to a particular color. You paint in each shape and ultimately the picture emerges as a finished painting. The paint by numbers approach is often ridiculed as being simplistic, uncreative, and formulaic. I believe it's helpful in getting across the concept that a painting is built up through multiple shapes of color. http://www.kshomediy.com/paint-by-numbers/