The "Let's Talk About..." series of books was created in a school classroom. At first, we started playing with numbers, trying to find things that are always odd or even, animals with four or eight legs, fairy tales with heroes that are three or seven, and songs that mention specific numbers.
So gradually, I began to understand that arithmetic is much easier when you sing it. The ten songs that were created resulted in the creation of ten colorful booklets, one for each number in the first ten.