Modern life makes us tired, right? But research from societies in Africa and South America suggests people in the ancient world slept only 6.5 hours a night.

Modern life makes us tired, right? But research from societies in Africa and South America suggests people in the ancient ...

美国20日将举行总统就职典礼，传统上总统交接有不少独特仪式，卸任者会给继任者留笔信、就职前共进午餐，卸任者还会拿到就职与卸任当天在白宫悬挂的国旗留念。