What if I told you that before you drink you beer you have to dump bugs into it to make the beer taste just right. And not just a few bugs. Billions of bugs.
Without yeast beer will taste like watered down syrup. Worst of all: without yeast beer won't have any alcohol. *gasp* But how? Why? Well I'm going to tell you.
From what I've read (as far as I can tell and like many things) beer was a mistake. The earliest tales I've read about beer brewing date back to the vikings. Each Viking family had a "brewing stick" which was an old piece of wood (for imaginations sake, taken from a viking vessel after pillaging hundreds of villages) that was used to stir the beer-soup. This was not a clean piece of wood mind you; it was probably some burned, bloody, feces covered (or just dusty, cob-webby, mold covered) piece sitting in the corner of the hut. Vikings lived in huts right?
The idea is that every family had their stick and every family had their own beer. The magic was in the stick. Without the stick the beer wouldn't have any good flavor or the magical effect that makes you stumble around like an idiot. The real magic was the fact that each stick had its own strain of yeast growing on it. Every culture (group) of yeast had its own flavor. This is why you can buy so many different kinds of yeast from your brew store today.
For most of the brewing process you're setting the grand ol' stage for the yeast. Unlike setting up a real rock and roll stage this has to be a very clean process, void of sweat, blood, and cocaine. Mashing the grains and the boiling process break down sugars into simple sugars that yeast uses to first replicate itself, and then to produce alcohol.
Yes, I said it. Yeast has sex in your beer. But only in the scientific term. Yeast replicates asexually (with out a mate; boring) by budding. Imagine every time you got a pimple it turned into a baby. Freaky right? The key here is that yeast reproduces quickly in the presence of oxygen. That's why you oxygenate your beer when you pitch (dump) your yeast into your wort, to introduce as much oxygen into your beer as possible. It makes them nice and healthy to get them ready to do their real business.
When oxygen is present, the byproduct of the yeast-sex is CO2 (the fizz in your beer). When the oxygen is gone, the yeast still produces CO2 (along with more of itself), but also produces *dun dun dun!* alcohol. How well it coverts sugar into alcohol is called attenuation. And it keeps doing this until... well, until you say it's done really. Much like wine you can cellar bottles of beer with yeast still in the bottom. As long as a beer isn't pasteurized there is very likely some yeast sediment in the bottom. Don't drink it though: from what I understand it gives you the runs. So unless your name is Ray from a 90's commercial for Philips', don't do it.