The story in three lines. The last span needs eight planks, but the Commons store is empty and people are keeping their spares in their sheds. Lan writes a rule on her own (everybody must give two planks), and it fails, because it would take Vera's only plank and nobody agreed to it. The island meets in a circle where everyone has a turn, adds "never your last" and an open book of what goes in and out, and agrees the rule twelve of twelve. The bridge is whole: FAIR, LIKELY, ENOUGH, OURS.

One question to ask. "Lan's rule was clearer than the island's rule. Why didn't it work?" There is a lot in the answer: nobody agreed to it, it was unfair to people with only one, and nobody could see whether others were giving.

One thing to do at the table. Pick one small shared thing at home that causes squabbles: the biscuit tin, the remote, the good pencils. Hold a meeting. Everybody gets one turn to talk without being interrupted, the youngest included. Write down only what people actually said. Then vote. Try the rule for a week, and meet again to see whether it needs changing.

The grown-up layer. For a long time, many economists believed that anything shared would be used up, because each person takes a bit more than their fair share: "the tragedy of the commons". The political economist Elinor Ostrom spent her career studying real shared resources, such as fisheries, forests and irrigation systems, and found that many communities manage them well for centuries. She won the Nobel Prize in economics in 2009. Among the things that tended to work: the people who use the resource help make its rules, the rules fit local conditions, and use is watched openly. The four words on the bridge make a small version of the same idea.

Let your child tell the story back in their own words first. Telling it back is how a story becomes theirs.