The story in three lines. Brimwater's new bell must cross on a ferry that can carry 400 kilograms, and no scale can lift it. Inkling guesses by looking, and Glug the frog stops her: a guess is not a weight. The children float the bell, scratch the waterline, swap the bell for river stones until the water sits on the same line, and weigh the stones twenty kilograms at a time: 352 kilograms, safely under.

One question to ask. "Why did they take stones out again, when they were nearly there?" Nearly the same mark means nearly the same weight, and the ferry needed the real number. If your child says "because Inkling said not yet", ask what would have gone wrong.

One thing to do at the table. Float a small plastic tub in a bowl of water. Put something in it that won't fit on your kitchen scale, or that you want to weigh without a scale: a toy, a shell, an orange. Mark the waterline on the tub with a pen. Take the thing out and add coins, marbles or spoonfuls of rice until the tub sinks to the same line. Now weigh those, or simply say "the orange weighs twenty-three coins". Try two things and guess first which will need more.

The grown-up layer. A floating object pushes aside its own weight of water, so the same boat sitting at the same waterline is carrying the same weight, whatever the load is made of. The trick is a substitution: when you can't measure a thing directly, find something equal to it that you can measure. Vietnamese children meet it at primary school (today in third grade), in a lesson called Cân voi (Weighing the Elephant), told about the fifteenth-century mathematician Lương Thế Vinh; Chinese children hear the same story about Cao Chong. The man is real and the story is a giai thoại, a tale told about him. The counting step is place value: seventeen baskets is one ten and seven, so the weight is ten twenties and seven twenties.

The series. This is the fifth and last book of The First Road. The tiny hammer the children find in every town is the mender's mark. Back on the Green it turns out to be the Sparks' grandmother's, and every child's who rode the cart before her. When Inkling rings the hammer for Lan, she becomes the older child for somebody else. You might ask: who rang it for you?

Let your child tell the story back in their own words first. Telling it back is how the scriber goes on the belt.