The village green feeds everyone’s cows. You have 10; your three neighbours have 30 between them.
Each neighbour buys more cows whenever the grass has been looking good, because the milk is theirs but the grass is shared. They sell only slowly when it looks poor. You can choose your own herd, and you can ask the village to agree a rule.
Goal: Get as much milk as you can from your own cows over 30 years, without the green ever being grazed below 150 tonnes.