In some forests, if you look straight up, the treetops stop just short of one another, and the canopy reads like a map of countries with rivers of sky for borders. Foresters call it crown shyness, and people have been writing about it since the 1920s without agreeing why. In a windy stand of black mangroves in Costa Rica, measured in 1984, the gaps were wider where the trees swayed further, so there the neighbours seem simply to wear each other’s edges off. But in Malaysian camphor trees nobody has found the scuffing, and the best guess is that the growing tips sense a neighbour’s shade and stop. So the same polite gap may have been invented more than once, by different trees for different reasons, the way two families can teach their children the same manners without ever having met.