Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
Why is the sky blue, and sunsets red? (for a curious adult who has never studied physics). Cover, in this order: What is white light made of?; What does air do to a beam of light?; Why does blue light scatter more than red?; So why isn't the sky violet?; Why are sunsets red?; Why are clouds white? — page 5: Why Sunsets Look Red
Noon: the sun is overhead, its light crosses a thin slice of air, loses a little blue, and still looks white.
Now the sun sits on the horizon. Its light must slant sideways through the air, crossing far more of it before it reaches you.
Mile after mile, air flings blue light off to the sides. Green and yellow weaken too, but red barely gets touched.
What arrives is sunlight with the blue subtracted. Take blue out of white, and what's left leans orange and red.
A blue sky and a red sunset are the same effect: air scatters blue first, so you see blue in the sky overhead and red in the sun that's left.