Why does the sky turn red at sunset?
The Real Answer
The sky turns red at sunset because of how light scatters through the atmosphere. Blue light has a shorter wavelength and scatters easily, so during the day it fills the sky. At sunset, sunlight travels through more atmosphere at a lower angle, and the blue light scatters away before reaching you. The longer-wavelength red and orange light makes it through, painting the sky. Particles like dust and pollution actually enhance the effect by giving the light more to scatter off of.
The sun isn't actually changing color — it's just that all the blue light gave up and left, and now you're looking at the leftovers. We've built our entire aesthetic of 'nature's beauty' on the fact that the sun is leaving and taking the good light with it.
Why People Ask This
This is usually asked by someone who just stopped and actually looked at a sunset for the first time in years — which is rare because we're mostly staring at our phones. Once you notice it, it's impossible to stop wondering. Most people assume it's the sun turning red, when really it's more of a cosmic game of musical chairs where blue light loses every time.