The Sand/Light/Glass theory is something that I came up with in college. It is a way of explaining how a person grows over time. It is really an extended metaphor, not so much a theory. Here it is:
Human experience is like a pile of sand. When you are born you are given some sand from each of your parents, from that moment on you accumulate more and more sand every day, every minute even. Each person you meet, each event in your life, adds some sand to your pile. Imagine it as your own, personal beach. When good things happen to you or you meet loving people, you might gain a certain color of sand, maybe green. When bad things happen to you, you might get red sand. As you go through your life getting more and more sand, a color will come to dominate your pile. You might have a beach of green or a beach of red.
Now, babies are like vapor. They are light and clear floating here and there. As the baby grows older the vapor condenses more and more until it becomes water. A child is water. Playful and flowing all over the place. As the child grows older, accumulating experience (sand) they begin to fuse this sand with their natural light (perhaps the soul). Fire and sand creates glass. The child is becoming a person, their inner light interacts with their experience, creating glass. This glass encases the water of childhood and becomes the lens they will see the world through.
Imagine this in your mind. Imagine a glass body filled with water. This is a teenager. They are stiff and awkward on the outside, but still like children on the inside. They still want to play and be free. The glass is the filter that they view the world through, the lens. It's largely formed by childhood experiences. This is why teenagers are often dogmatic in their views on life. This glass lens distorts our view of the world and limits our vision.
Here is an example of how the lens works: I grew up in the California suburbs, my childhood experiences caused my glass to form in a certain way. I had very definite views on personal hygiene that, especially as a teenager, I thought were absolute and true for everyone everywhere. Then I went into the Peace Corps and lived in a village where my views totally changed. It became clear that things were not objectively "gross", I had chosen to see them that way. My lens had made them seem that way.
Around age 17 or 18, depending on the person, the outside world begins to tap on the glass. The teenager realizes that the world is not their playground as it was in childhood. Events that trigger these realizations hit the glass and cause cracks. The more intense or traumatic the experience, the bigger the crack. Through the cracks small glimpses of truth become visible.
A good example of an event that would cause a crack would be September 11th. When the towers fell, Americans were shocked, not only by the events themselves, but by the motivation. Many people had no idea that America was so unpopular in certain circles. This created a huge crack in the glasslens that we view the world through, suddenly we could see a new bit of truth about our world.
Another consequence of the cracks in our glass shell, is that it allows the liquid of our childhood drain out. Soon we start to feel empty and try to fill the space with something, anything to feel full again. Some people become workaholics, some drink, some start collecting sports memorabilia obsessively, some join the Peace Corps. It's different for everyone, but we all try to fill our empty shell with something to keep from having to look through the cracks in our lens at the truth.
In the example of 9/11, America collectively started to fill itself with patriotic rhetoric to keep from having to look at the reality of the situation. We no longer cared what happened or why, all that mattered was how proud we were to be Americans. Now we have the Iraq war to fill ourselves with.
The cycle never ends, even as we patch up cracks in our glass with nonsense, new ones appear. We continually drain and feel empty, needing to earn more, buy more, do more just to feel full for a moment. The only way to break the cycle is to stop filling ourselves with nonsense long enough to recognize what is really happening. Really look through the cracks and see the truth, whatever it may be for each of us. Then we may be strong enough to take a hammer and break what's left of the tired glass we created to protect ourselves from reality. If we can truly shatter our glass for good, we may find freedom.