There are a few things US students love more than summer vacation, that traditionally three-month break from school during June, July and August. But why does it even exist? A common theory holds that summer vacation was created for farm kids who needed to work. That’s only marginally true: students at rural schools did get time off for harvests, but those breaks fell in the spring and the fall. In fact, summer vacation was born in the city. Calls for a break began in the mid19th century, when many urban schools met year-round. Warnings about “overstudy” led reformers like Horace Mann to argue that time off would be good for kids’ behaviour and mental health, says Kenneth M. Gold, author of School’s In: The History of Summer Education in American Public Schools. Doctors agreed: John C. Warren, a Harvard professor of anatomy and surgery, once remarked that “too steady an application to literary pursuits” led to a “debilitated body.” And in an era before AC, experts worried that poorly ventilated school buildings compounded the risks. Debate raged for decades. But as railroads made nature more accessible for city dwellers, wealthier parents increasingly took matters into their own hands and took their kids out of school. Eventually, the vacationers won out, and the break became official. Nowadays, reformers have changed their tune: they argue that eliminating long summer vacations could shrink achievement gaps between students. It’s an idea backed up by data—but one at which their forebears might shudder.