The Dork Circus
All know that the natural world has two dark reflections: The Shadowfell and the Winterdark. Parents everywhere scare their children into behaving by threatening them with the possibility that if they are naughty, they will wake one morning in the Winterdark, where the sun never rises and it is always winter, but never Christmas.
Thin places in the world, where the Winterdark comes too close to the natural world, are truly the only places one can crossover between planes. But a child need not know that.