Maybe you've experienced it before. You're walking through a forest and suddenly you hear the guttural croo croo croo of a raven flying by. At that moment, everything seems to stand on its hind legs. The forest you're walking through is no longer the same forest. It has changed. The needles on the pine trees are pricklier than they were a few moments ago. The bark of the trees is covered in a shiny crust. The mosses have become richer...