Commonly, kids will ask, "why is the sky blue?" and not knowing the real answer, parents will answer something like "well it's the reflection off of the oceans" or "that's just the way it is". Little do these kids know, they are actually inquiring about physics.
In reality, beautiful blue skies are more then just a nice day or a pretty sight, it is actually only blue because that's the way the human eyes interpret it. When our eyes perceive colors, all they are actually doing is distinguishing different wave lengths of visible light when they strike our eye. It is possible for the wavelengths to be straight from a light source, like the sun, or reflected off of another object.
The Earth's atmosphere is made up of tiny gas particles that are much smaller then wavelengths. When light hits a gas molecule, some of that light gets absorbed. Because they have shorter wavelengths and much higher energies and higher frequencies, blues and violets tend to get absorbed into the particles before the lower frequency colors. The gas molecule then releases the light in the color of the wavelength that it absorbed.
From there, you may ask yourself, "why doesn't the sky appear violet then?". This is because your eye's color interpreters called 'cones' average the wavelengths together so that eyes will only perceive one color. The eye interprets the sky as mainly blue and white light (or visible light from the sun) and therefore the sky is light blue.
Red sunsets are also explained by this phenomenon. Because when the sunsets, it gets further and further from the one who is viewing it, it takes longer wavelengths of visible light for the eye to see. Because the red side of the spectrum has much longer wavelengths, the eye perceives sunsets as red.