Have you ever looked up to the sky and wondered, what is it that makes the sky blue? It's one of them things where you accept it and move on but at the root of it, what is causing this?
Generally speaking, different molecules are able to scatter different wavelengths of electromagnetic waves. So when the sunlight reaches earth, since its light is white therefore is composed of all colours, the molecules that make up our atmosphere scatter the wavelengths of blue light much more than red light due to the chemical properties of the molecules. Hence blue skies.
Here is a video that answers this question in depth and in case the above didn't make sense to you.
Mystifact