Why is the sky blue?

All the light in our daytime sky comes from the sun, and our sun produces light in many different colors. To understand this, just think of a prism.  White light enters, and a rainbow of colors exits. Our sun is like that. Its light may seem uniform but it is actually emitting many different colors.

Our sky though is not a mix of blue and red and green because the earth's atmosphere is sandwiched between the sun and us, getting in the way. Atmospheric gases and particles scatter the incoming colors, bouncing them in all different directions.  The amount of scattering increases as the wavelength of light decreases.  Red light which has a long wavelength is scattered much less than blue light which has a short wavelength. As blue is scattered more, it fills our daytime sky.

As the sun traverses the sky and moves low on the horizon, the light that reaches our eyes must actually travel further through the atmosphere.  This has the effect of completely bouncing away all the blue light.  We are left with the reds and yellows and oranges in wonderful mixture, making sunrises and sunsets so beautiful.