When you count to 20, you start at 1 (or zero) and count on until you reach 20. Each number is one more than the last. When you count back from 20, you start at 20 and count back until you reach 1 ...

Joeykentin/Wikimedia Commons, CC BY-SA Informally, a positive counting number larger than one is prime if that number of dots can be arranged only into a rectangular array with one column or one row.

Findings of a study suggest using a ‘hundreds chart’ showing numbers one through 100, beginning with one in the top-left corner, fosters children’s counting by 10s.