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Why is the sunset red?

Because the blue has already left

In the evening, its light comes in from the side and crosses a layer of air that's much thicker

The molecules in the air scatter the blue in every direction

The red passes straight through

And some evenings turn blood red: after an eruption, the sky of the whole world changed for years

The sunset isn't redder: it's just the blue that has reached the end of its road