Astrophysicist explains one concept in 5 Levels of difficulty
With their feet dangling, and amusing themselves–until I stopped them–by throwing stones at the giant mass. After I had spoken to them about it, they began playing at “touch” in and out of the group of bystanders. Among these were a couple of cyclists, a jobbing gardener I employed sometimes, a girl carrying a baby, Gregg the butcher and his little boy, and two or three loafers and golf caddies who were accustomed to hang about the railway station. There was very little talking. Few of the common people in England had anything but the vaguest astronomical ideas in those…