Why do trees sway?

G K Chesterton once said " ..that there are two kinds of people in the world: When trees are waving wildly in the wind, one group of people thinks that it is the wind that moves the trees, the other group thinks that the motion of the trees creates the wind. The former view was the one held by most of humankind through most of its centuries; it was only in recent years that a new breed of people had emerged who blandly hold that it is the movement of trees that creates the wind. The consensus had always held that the invisible is behind and gives energy to the visible; ' Chesterton reported with alarm ...' that the broad consensus had fallen apart and that the modern majority naively assumes that what they see and hear and touch is basic reality and generates whatever people come up with that cannot be verified with the senses. They think that the visible accounts for the invisible'