The sizes of leaves vary by more than five orders of magnitude among plant species worldwide, yet the reason why has remained mysterious. Ian Wright of Macquarie University in Australia and colleagues have now characterised the patterns of leaf sizes for 7670 plant species together with climatic data from 682 sites worldwide, and report consistent patterns that explain why earlier predictions had limited success. The results also provide a quantitative explanation for leaf-size variation with latitude: for much of the world, a key factor limiting leaf size turns out to be night temperature and the associated risk of frost damage.