Six school supplies, from a pencil to a box of crayons, each have a price tag and a row of coins beside them. The child counts the coins, writes the amount, and circles yes or no. Three rows come out yes and three come out no.
The no answers are the instructive ones, because two of them miss by very little: one cent short for the pencil, four cents for the crayons. A child has to count carefully to catch them. The ruler is an exact match, which is a good chance to talk about whether having exactly the price counts as enough. It does.