The story in three lines. The spark takes Lan and Inkling to a mud-brick town beside the Nile, about 3,900 years ago, where a family must lift fifty baskets of bricks onto their roof before the flood season begins. Pulling up hand over hand is slow; a rope over a fixed beam lets them pull down, but it rubs, squeals and frays. Lan asks whether the round thing could turn with the rope, and the uncle carves a grooved wheel; Inkling asks for a second wheel on the basket, which halves the pull. The four letters on the anvil now read DOOR.

One question to ask. "Why was pulling down easier than pulling up?" Listen for "you use your whole weight" or "like hanging." Then ask: "What did the little wheel stop the rope doing?"

One thing to do at the table. Thread string through a cotton reel on a pencil, held by a grown-up at each end. Tie a small bag of coins to one end and pull the other end down. Then try the string over a pencil with no reel, and feel the difference.

The grown-up layer. A fixed pulley changes the direction of a pull, so you can use your body weight; a wheel that turns with the rope replaces sliding friction with rolling. A pulley riding on the load is held by two parts of the rope, so each pull is half as hard, over twice the distance. The earliest evidence of pulleys comes from Egypt's Twelfth Dynasty and from Mesopotamia; nobody knows who made the first. Across all four books the same rule holds: the Sparks bring questions, and the people of the past do the making.

Let your child tell the story back first. Telling it back is how the last letter goes on the anvil.