A question from a five-years-old: why is sky blue?

A correct answer: you need to learn how to read, then some mathematics, then quantum physics, then you can calculate how sunlight scatters on molecules of air. Understanding the physiology of color vision won't hurt, too.

This is not very useful. But at least you see some topics to learn more about, and a simplified picture may be built: sunlight contains some blue light, and it gets stuck in the air, while reds and greens pass in more easily. Hey, you just understood why sunsets are orange and red! Go on.