The story in three lines. Bà Tư's party jelly is still liquid the night before, and Lan has promised the guests a wobbly one. Inkling blames the fridge, but Compare says there are three suspects (the fridge, the water, the fresh pineapple), so they make three small bowls and change one thing in each. By morning only the bowl without pineapple has set, and Bà Tư shows that pineapple from a tin, or fresh pineapple simmered first, sets fine.

One question to ask. "How did they know it wasn't the fridge?" Because the bowl that only changed the fridge stayed runny. If your child says "because the pineapple one set", ask: what was different about that bowl, and what was the same?

One thing to do at the table. You need a packet of gelatine jelly, three small cups, a little fresh pineapple and a little tinned pineapple. Make the jelly with a grown-up. Put nothing in cup one, fresh pineapple in cup two, tinned pineapple in cup three. Same jelly, same fridge, same time. Guess first: which will set? Check in the morning, and poke all three.

The grown-up layer. Gelatine is protein, long strands that tangle as they cool and hold the water in a wobbly net. Fresh pineapple contains bromelain, an enzyme that cuts those protein strands, so the net never forms. Canning heats the fruit, which unfolds and stops the enzyme, so tinned pineapple is safe for jelly; so is fresh pineapple that has been simmered. Kiwi and papaya do the same. Agar jelly, made from seaweed, is not protein at all, so bromelain has nothing to cut. The story stops one step short of the word "enzyme" on purpose: below nine we let Clippa's scissors carry the picture.

Two things the book is really about. Change one thing at a time. And the gray answer: pineapple isn't bad, it's wrong for this job, and fresh and cooked are not the same. Lan's part is worth a word too: she learns to check before she promises.

Let your child tell the story back in their own words first.