I've heard that it is the way light reflects on the dust particles in the air. I used to think it was because the universe is black and dark and it appears blue until it fades to black. I never researched this answer for myself and I don't remember learning about it in school.
What happens when I die?
You obviously lose your identity in the material world and therefore unite with a consciousness next of kin, the astral body. Our Astral body is with us while alive as well. Upon bodily death your Etheric energy that animated your body will detach and dissipate, you'll experience rest and acclimate to new consciousness, then experience a run down of the Karma you've made throughout that lifetime instantaneously, in a time period so infinitesimal it defies rationale. Faster than seeing a two hour movie in the blink of an eye. You will work in this Astral consciousness, much like you worked in your physical lifetime. You'll all the while be doing the business of your soul, but you will still work within your own free will, choosing to do this or that or not. The Astral consciousness mirrors what you knew on Earth, and perception of people, spirits, places, situations are much like that in a dream, 'seen' in lights and colors, felt in emotions. Knowledge is developed and once you have done the work that needs to be done in this consciousness, working out karmas, learning lessons, healing, helping, working, then you'll reincarnate into another physical body on Earth. This process generally takes no less than what 100 years time is on Earth. Reincarnation will put you with the same members of your previous life's family including ancestors. Think, you have most likely been your mother's mother, your nephew's great grandfather or his wife. This will maintain until members outgrow the vibration of this family and then they will move onto another family where their vibration is now in sync with. This process of life and death will repeat until we reach our Soul level where we will be working in another form of energy, consciousness and vibration and we have utilized our need for physical manifestation. Then, there is and ever will be more to become. Life and Death remain mere concepts born of our perceptions at this time but their reality is much more expansive than that. Life and Death are devoid of our emotions, our justice, our flesh and bone and sinew. We are consciousness, We are energy, We are one and We are infinite.
"Just as a caterpillar, having reached the end of a blade of grass and approaching another one collects itself, even so this atman, having discarded the body and overcome ignorance, approaching another one collects itself. That to which the heart is attached, toward this the subtle body moves together with its action which still adheres. Attaining the goal of whatever actions he performed here on Earth, he goes once more from that world to this world of action."
Shukla Yajur Veda, Brihadaranyaka Upanishad, 4.4.3 & 6 VE, P. 759
For more information along the lines of what I've written go see: http://www.himalayanacademy.com/resourc
-Helena Blavatsky Writer of The Secret Doctrine
What is sex?
Besides the uniting constant for humans that this is our means to reproduce, sex will mean different things to you at subsequent times in your life. It's amazing its ability to shapeshift before you can exactly put your finger on it, or in it.