It has been long recognized that in Western countries, children are more likely to become overweight or obese over the summer. Causes of this include changes in kids’ physical activity and diet over during the summer period, including the summer holidays. But in a new study in Frontiers in Physiology, scientists from the US show that this “obesogenicity” of summers has another unexpected cause: children grow faster over the school year than over the summer. And because Body Mass Index (BMI) is the ratio of body weight in kg and height in meters squared, faster vertical growth during the school leads to increased BMI during summers.