Build a loop. You need one AA battery, a torch bulb or an LED, and two pieces of foil about a finger long. Hold one end of foil on each end of the battery and touch the other ends to the bulb. It lights. Lift one strip away: the loop is broken and the light goes out. (Batteries only. Never the wall socket.)

Before chapter 3. The batteries are fine, and a new bulb does nothing. Where would you look next?

The answer. Somewhere the loop is broken. Follow it: battery, wire, switch, bulb, back. In the book a wire has slipped out of the switch. Three 1.5-volt batteries in a row give 4.5 volts, so the push was never the problem.

Pause to think. There are two things that stopped working in this book. What was the gap in each one, and who closed it?

The grown-up layer. Children often blame the battery or the bulb because those are the parts they can see. Tracing the whole loop is the habit. Repair Cafés, which began in Amsterdam in 2009, are a good outing if you have one nearby. CircuitForge has the loop in a game.

For the ride home. Ask: what at home has a switch? Where does its loop go?