Ask a grown-up why the sky is blue and watch them hesitate. Most of us learned the answer once and then quietly lost it. Here it is again, in a form you can hand straight to a child.

Sunlight is not yellow

It looks yellow-white, but sunlight is really every colour mixed together. You have seen the proof: a rainbow is sunlight taken apart into its pieces. So is the little band of colour that appears on the wall when light catches the edge of a glass.

Hold on to that idea, because everything else follows from it.

The sky is full of things too small to see

Air is not nothing. It is packed with gas molecules — mostly nitrogen and oxygen — far too tiny to see, but there in enormous numbers all the way up.

When sunlight arrives, it crashes into them.

Blue bounces the most

Here is the part that does the work. Light travels in waves, and the colours have different wave sizes. Red waves are long and lazy. Blue waves are short and quick.

Those tiny air molecules are much better at knocking the short, quick waves off course than the long, lazy ones. So blue light gets scattered — bounced sideways, ricocheting from molecule to molecule all over the sky — while red and yellow mostly sail straight through.

That is why the blue seems to come from everywhere rather than from the Sun itself. You are seeing light that has been thrown around the sky before it reached your eyes.

The one-sentence version: the sky is blue because blue light bounces off the air more than any other colour, so it arrives at your eyes from every direction at once.

Then why is the sunset red?

This is the question a curious child asks about four seconds later, and it has a satisfying answer.

At sunset, the Sun is low, so its light has to travel through far more air to reach you — a long slanting path instead of a short one straight down. Along that extra distance, nearly all the blue gets scattered away before it arrives. What is left to reach your eyes is the light that never bounced: the reds and oranges.

Same sunlight. Same air. Different amount of it.

Try it at the kitchen table

You can show this with things you already own.

  1. Fill a clear glass or jar with water.
  2. Add the tiniest drop of milk and stir — you want the water barely cloudy, not white. The milk droplets stand in for air molecules.
  3. Shine a torch through the side of the glass in a darkened room.

Look at the glass from the side: the water has a faint blue-grey haze. That is scattered light — the sky.

Now look straight down the torch beam, through the water towards the light. It turns warm and orange. That is your sunset.

If it does not work the first time, add less milk, not more. Almost everyone overdoes it.

The question behind the question

When a child asks why the sky is blue, they are really asking whether the world makes sense — whether there are reasons underneath ordinary things. There are. And “let’s find out” is a perfectly good answer when you do not know one.

This is the kind of question that fills Why? The Big Book of How Things Work. If your child asks a lot of them, that book was made for your house.