In Summary

So if any one asks you why the sky is blue just tell them, "It is because Rayleigh scattering causes photons traveling at a "blue" wavelength to scatter throughout our atmosphere, and the inability of the cones in our retina to detect violet light."

Or just tell them the sun's light contains a rainbow of colors, even though we perceive it to be white light. Then when the sunlight enters the earth's atmosphere the blue light gets scattered throughout the sky making it appear blue. Meanwhile the rest of the colors travels straight to your eye as white sunlight. 

But don't take our word for it, just click here for some cool scientific experiments you can do on your own.