Why is the sky blue?
Go outside and look at the sky.
If it's daytime, and it's clear, chances are you're looking at a wonderful, deep shade of blue. But why?
When sunlight passes through the atmosphere, some of the light hits the oxygen and nitrogen molecules that make up most of the air. Blue light has the shortest wavelength, and it is it the first to scatter. It scatters off the air molecules, filling the sky with a beautiful blue color.
So why is the sky yellow and red in the evening?
Well, during the daytime, light is coming straight down through the atmosphere, and the sky is filled with blue. But in the evening, light is coming in from near the horizon and has to travel through more atmosphere to get to you. Because blue light scatters out first, most of the blues are scattered out by the time the light gets to you. All that's left are the yellows and reds.
It's amazing to think that someone else's blue skies are making yours red!