The phenomenon eddie's talking about here is Rayleigh Scattering, if you want to look for a mathematical description of it on the net (which tells you more if you can understand the maths).
Why is the sky blue?
Easypeasy. It's all down to the scattering of light. Photons normally travel in straight lines, but when one hits a nitrogen molecule or another particle in the Earth's atmosphere, it undergoes a 'scattering' process, i.e. some of it's energy bounces off in a different direction. The actual direction is determined by the actual wavelength of the light, the size of the particle it hits and so on. For the makeup of our atmosphere, this effect is greatest in the shorter (blue end of the spectrum) wavelengths, and that's why the sky appears to be blue.
The short answer is that no-one know 'why' it happens. We've just been able to observe it. To 'watch' any biological phenomena, you just have to radioactively label a certain peptide (like one of the RNA's for example) and watch where it goes and what happens to it. I personally watched as proteins get synthesised at the ribosomes and it's a pretty dull and slow process. :D
how do scientists know that protein synthesis happens by the unzipping of dna, followed by the mRNA transcribing the information and the tRNA translating it and it being turned into amino acids in ribosomes in our cells.