Make a sinking-bowl clock. A grown-up pokes one small hole in the bottom of a plastic pot or cup. Float it in a bowl of water and time how long it takes to sink, using a clock. Do it three times. Is it the same each time? Try a bigger hole.

Before chapter 3. Why can't Lan time the bowl by counting "one elephant, two elephant"?

The answer. Counting in your head speeds up and slows down, and it is easy to lose your place. A clock needs something that happens at a steady rate. The copper bowl in the book sinks every 24 minutes, and sixty sinkings make a day: 60 × 24 = 1440 minutes, the same as 24 hours of 60 minutes.

The honest part. Nobody knows when the very first sinking bowl was made, or by whom.

The grown-up layer. Sinking bowls were used in India (the ghaṭī) and in Persia, where they timed shares of water for farms. MeasureQuest has Cup (volume) and Tick (time).

For the ride home. Ask: what else happens at a steady rate?