Imagine you're outside and it's getting dark and cold. You look off in the distance and see a small light. You move toward the light and find it's coming from a cave. As you draw nearer you feel warmth coming from the cave. You enter and discover that you're in a huge cavern with food all over. You gorge yourself and find a resting place. Then the light goes out!
Eventually the cavern is bright again, but the light is different. Instead of coming from the roof of the cavern, the light is coming from the sides. There are big holes in the sides of the cavern, and you see sunlight coming in from these holes. You go over to one of the holes and find your way blocked by a solid, invisible substance. You feel all over the surface of this substance, but can find no way past it to the outside which you can see so plainly.
You make louder and louder noises of frustration. Suddenly, you see a large flat object coming right at you. You quickly dodge it and go to another opening. Again you are blocked by an invisible substance. Where, oh where is the hole you came in? Again, you make noises of...
