Sunlight looks white, but it has every colour mixed inside it. When it shines through the air, the blue bounces around much more than the red and yellow. So when you look up, blue light is coming at you from all over the sky.

Why does blue bounce around more?

Light travels in tiny wiggles, and each colour wiggles differently. Blue has short, quick wiggles. Air is full of bits too small to see, and short wiggles keep bumping into them and bouncing off. Red's long, lazy wiggles mostly slip straight past.

Then why isn't the sky purple?

Violet wiggles are even shorter than blue, so violet bounces even more. But sunlight has less violet in it, and our eyes are much better at seeing blue. So the sky looks blue to us.

Why is the sky blue?

Roo answers for ages 4–11, then goes 2 steps deeper each time a child asks why.

Roo says

Sunlight looks white, but it has every colour mixed inside it. When it shines through the air, the blue bounces around much more than the red and yellow. So when you look up, blue light is coming at you from all over the sky.

For ages 4–7

Checked against Why is the sky blue? (NASA Space Place)
Why does blue bounce around more?

Light travels in tiny wiggles, and each colour wiggles differently. Blue has short, quick wiggles. Air is full of bits too small to see, and short wiggles keep bumping into them and bouncing off. Red's long, lazy wiggles mostly slip straight past.

For ages 6–9

Checked against Why is the sky blue? (NASA Space Place)
Then why isn't the sky purple?

Violet wiggles are even shorter than blue, so violet bounces even more. But sunlight has less violet in it, and our eyes are much better at seeing blue. So the sky looks blue to us.

For ages 8–11

Checked against Why is the sky blue? (Met Office)