Try it in a sink or a bath. You need a plastic tub or a takeaway box that floats, a strip of tape, a small heavy toy, a handful of pebbles or coins, and a kitchen scale. Stay close to children near water.

Before chapter 3. Float the tub. Put the toy in. Mark the waterline with tape. Take the toy out and add pebbles until the water touches the tape again.

Before chapter 4. Weigh the pebbles in a bowl. Then weigh the empty bowl. What do the pebbles weigh on their own?

The answer. The pebbles weigh the scale reading take away the bowl. In the book that is 420 − 80 = 340 grams. Then weigh the toy by itself: it should be close to the same. A floating boat pushes aside its own weight of water, so the same weight always sinks it to the same line.

The grown-up layer. The same tale is told of Lương Thế Vinh in Vietnam and of Cao Chong in China, more than a thousand years apart. Nobody knows if either happened. The book files it under legend, and the mathematics holds either way. Subtracting the container (the tare) is a habit worth building early. MeasureQuest has a scale game; BackstageQuest has the condition report.

For the ride home. Ask: what else is too big for a scale? How could you weigh it?