There’s a happy-making little film on YouTube of a man dancing by himself at a music festival. Some people sitting on the grass nearby look on, curious and amused. Most ignore him or don’t notice him; their backs are turned and their attention elsewhere. Then after a minute or so, another person gets up and joins in, grinning and a bit self-conscious, but with him nevertheless. Now there are two people dancing. Another stands up, hesitates and then starts to dance. Now there are three: three makes it a group. There are four, five, 10, more and more. Too many to count.And soon a whole field of people is dancing. It’s become a movement. The people who started it don’t matter any more.