The story in three lines. Lan and Inkling arrive in ancient Egypt the day before a giant statue must reach the river, and they cannot simply tell anyone the answer. Pulling harder on dry sand fails, and a spilled jar half-works before turning to mud. Then a footprint shows them that just-damp sand is firm, and Lan asks the water girl a question: what if she pours a little in front as they go?

One question to ask. "Why didn't more pullers help?" Listen for the heap of sand in front of the runners. Then ask: "Why was a little water better than a whole jar?"

One thing to do at the table. You need a tray, some dry sand (or sugar, or salt) and a small flat block, like a matchbox. Push the block across the dry sand and watch what piles up in front. Then sprinkle a few drops of water, just enough to darken it, and push again. Then try far too much water. Which slid best?

The grown-up layer. A real tomb painting, made for the governor Djehutihotep nearly four thousand years ago, shows a person pouring water in front of a sledge carrying a huge statue. In 2014, physicists found that the right amount of water roughly doubles the stiffness of sand, because tiny water bridges hold the grains together. Then the sand stops heaping up in front of the runners, and about half the pulling force is needed. Nobody knows the name of the person in the painting, or who first thought of it. You don't need any of this. "What is the sand doing?" is the whole idea.

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