The neighbor's kid asked me this morning why the sky is blue—and then immediately answered, "because of air." Close, but not quite. It's a perfect example of how the most everyday phenomena reveal layers of complexity once you slow down and examine them properly.
The blue of the sky comes from Rayleigh scattering, a process where shorter wavelengths of sunlight interact more strongly with molecules in the atmosphere. Blue and violet light have shorter wavelengths than red or orange light, so they scatter in all directions more effectively. We see blue instead of violet because our eyes are more sensitive to blue, and because some violet light gets absorbed higher up in the atmosphere. It's a subtle interplay between physics and biology—light waves and retinal cones working together to produce the color we take for granted.
Here's a quick way to visualize it. Imagine throwing a handful of pebbles at a chain-link fence. Small pebbles will ricochet off the wires in all directions, while larger stones might sail through the gaps. Shorter wavelengths behave like the small pebbles—they collide with air molecules and scatter. Longer wavelengths pass through with less disruption. That's why the sky overhead is blue, and why the horizon can look pale or even whitish when you're looking through more atmosphere at a shallow angle.