Finding that his brain was too addled to make any sense of the latest figures from Caltech on the continued exponential breakup of moon debris, he went to the "gym," which was a module containing several treadmills. To prevent their users from bouncing off them in zero gravity, these were equipped with waist belts and bungee cords that held the occupants "down," pressing their feet against the belt of the treadmill and forcing the legs to do some real work. Supposedly it was good for the bones and muscles. Amelia kept sending Doob emails asking him whether he’d exercised today, and he liked to make her happy by answering yes.