The sky is blue because the air scatters blue light from the Sun across the sky and down to our eyes.
We think of air as being clear and transparent, and it is. But air is made up of countless tiny objects, mostly molecules of nitrogen. When sunlight goes thousands of miles through our atmosphere, a tiny amount of light becomes scattered by these objects.
White light is made up of all colors of light. Nitrogen molecules scatter blue light more than they do the other colors. So a little bit of blue light bounces around in the sky and then enters our eyes. That makes the sky look blue.
Art by: iStock.com/CAEccles