Welsh 
Why is the Sky Blue?
You will need:
- A large, clear, straight-sided glass
- Water
- Milk
- A teaspoon
- A torch
- A darkened room
What to do:
Fill the glass about 2/3 full of water. Add half a teaspoon of milk and stir. In a darkened room, shine the torch down onto the top of the water whilst looking through the side of the glass. The light should appear blue. Then try shining the torch through the side of the glass whilst looking through the opposite side. The light appears yellowish. Finally, shine the torch up through the bottom of the glass and peer down through the water. The light should appear red. The milk particles in the water scatter the light from the torch like gas molecules in the atmosphere scatter light from the sun. The further the light has to travel through the water, the more of the blue light has been scattered, leaving only red light for you to see. Just like at sunset.
More Information
http://www.sciencemadesimple.com/sky_blue.html
http://www.ace.mmu.ac.uk/eae/english.html and
pick 'atmosphere' and 'blue sky'