Why is the sky blue?

Sunlight looks plain white, but it's really every color of the rainbow mixed together. When that light travels through the air, the blue part bounces around the most, all over the sky. So when you look up, blue is what fills your eyes.

Glossed over "Rayleigh scattering" and the fact that shorter wavelengths scatter more strongly than longer ones. "Blue bounces more" is a faithful shorthand for the feeling but doesn't yet introduce the wave model of light. Save the wave model for a deeper conversation.

Sunlight is a mix of every visible color, each traveling as a wave of a different length. As that light enters the atmosphere, the shorter-wavelength colors (blue most of all) scatter off air molecules far more than the longer ones; this is called Rayleigh scattering. We perceive blue rather than violet (even though violet scatters more strongly still) because the sun emits less violet to begin with and our eyes are less sensitive to it. A common misconception is that the sky is reflecting the ocean. Actually the causation runs the other way.