Comment on this: when you look at language under a microscope, you can see it changing almost as you watch it: words and phrases, pronunciation and rhythm change at astonishing speed.

Each language can be compared with a live organism - it's constantly changing over time, and this can be reflected in various aspects. The first aspect is vocabulary. Each year new and new words appear in a language. It is connected with the appearance of new things and new phenomena in the world, so each of these things should be given a name. Also more and more borrowings appear, taken from other languages. Pronunciation is the second aspect. Pronunciation very much depends on the speaking style and on the class of people speaking. So when the majority of native speakers start to pronounce the word with some deviations from the norm, this "corrupted" variant becomes a new norm. So one can never be sure that he masters the language perfectly, as new changes are constantly appearing.