The story in three lines. Bà Tư and the otters make a little cushion of four patches, two by two. It is too small for the cat. Broody the owl shows how a square grows: two patches along one side, two along the other, and one in the corner, and Tí lays them himself to make nine.

The five nights. Night one, listen. Night two, let your child say the bold last words. Night three, join in the refrains. Night four, stop when the cushion is too small and ask what to do. Night five, tell it to a toy.

One thing to do at the table. Make a square of four blocks, coasters or crackers. Now make it into a bigger square. How many more do you need? Then try again from nine.

The answer. Five more turns four into nine. Adding just one, on one side, makes a lopsided shape, not a square.

The grown-up layer. Growing squares by "two sides and a corner" is how square numbers work: 1, 4, 9, 16. PatchPals and ArrayPals build the same squares.

For the ride home. Ask: what else at home is a square?