The figures looked more or less human. And they were engaged in religion. You could tell by the knives (it’s not murder if you do it for a god).
—(Terry Pratchett, Small Gods)
You come upon a ruined building in the back section of a city park (or other out of the way area of the city). The ruins are fairly overgrown. All that is really standing is a doorway and its frame. If you pass through the opened door, you travel to a different world. If the door closes, there may not be a doorway back to get you back.