Make a glass sing. Wet a finger and rub it slowly round the rim of a thin glass, or tap a glass with a spoon. Each glass has its own note. Add a little water and the note changes. A radio tuner works the same way: it rings at one frequency, and the knob changes which one.

Before chapter 3. Beto turns the volume up so the right station will win. Will it?

The answer. No. Louder makes both stations louder. A radio picks one station by resonance: the tuner rings at that station's frequency and lets the rest go. In the book the dial cord had slipped off its wheel, so the knob did not move the tuner. On a medium-wave dial, 909 means 909 kilohertz: 909,000 waves a second, each about 330 metres long.

Pause to think. Something else was "hearing everything" at once in this book. What was it?

The grown-up layer. Old battery radios are a safe thing to open with a child; anything that plugs into the wall is a grown-up's job. WaveForge has resonance and frequency to play with.

For the ride home. Ask: what number is your favourite radio station?