Why is the Sky Blue?

My little sister and I were lying on the lawn, staring up at the changing clouds. In a moment, she released a large sigh and turned her face to me.

"Breanna?" she started, as if hesitant to ask, "Why is the sky blue?"

Such a simple question sprang to mind so many thoughts. At first, I went for a scientific approach.

"Well, the sky is blue because it reflects the ocean."

"Or is it the ocean that reflects the sky?" she added.

"Hmm." the thoughts in my mind were liquid, hard to grasp, "Then perhaps it is both. Maybe they are reflections of each other, of light bouncing off of one another that gives us the impression of the color blue."

It was silent for a moment before she propped herself up on her elbows and fully committed her attention to me.

"Well, then how does the light make it blue?"

"Well... what is color? I learned in physics that the color we see is really the frequency or wave of light for every other color there is; the one wave not present is the color we see."

I had even confused myself by then.

"But how can you see the absence of color?" she asked.

I wisely replied, "I don't know."

"Well, what is color then? How do I know that the blue I see really is blue? What if my blue is actually red but I was taught that it is blue?"

Silence. I had never thought of it before. Did the human eye really interpret things differently from person to person? Those who are colorblind certainly see things differently, especially when they can only see select colors, but whose to say they can see those colors? Is what I see as blue pink or even yellow to someone else? Is it some other color I cannot even imagine as I have never seen it? What is color?

"So," my sister interrupted my thoughts, "Is the sky really even blue?"