The Ten Commandments, or read the story in the bible, go check it out.
Each Passover, Jews all over the world stop eating leavened bread for the duration of the holiday (8 days) in sympathy with our brethren who fled Egypt without the time to let bread rise, as they were fearful of being pursued by Pharoh's soldiers and prevented from leaving Egypt. That the Jews ate matzoh then was driven by necessity, of course.
Today, the practice extends not just to bread itself, but to any food which might actually contain leavening (yeast or baking soda) such as breakfast cereal, or bagels, but also extends to those things that rise for other reasons, such as rice or corn. I bet you see where this is going! Beer is on the prohibited list, of course. Wine, which is also made with yeast, is not prohibited, AND we are commanded to drink four cups of it at the seder!
Doesn't seem fair, does it?
Even worse, beer (as are other "chametz" products--the link gets into some detail) are supposed to be disposed of before Passover starts. For someone with a beer fridge full of several batches of beer or expensive collected beer, this is just not an option! Luckily you can sell it to a non-Jewish friend, so there is no prospect of it being consumed, even by accident.
According to a conservative rabbi I consulted, the explanation of why beer is prohibited has to do with the fact that beer is made from malted barley, and the malting process causes the barley to be in contact with water for more than 18 minutes, which causes germination of the seed, and swelling of the husk (the 18 minutes is the significant bit). Thus it appears to me to fall into the same category as rice, as something that grows/swells not from leavening, but because water is introduced for more than 18 minutes. Hmmmmm.
One of the things I love about being Jewish is that EVERYTHING is open to interpretation and review. There is an old saying that if you put two Jews together, you will get three opinions. There are two branches of Judiasm (stick with me for a second, this will be clear): the Ashkenazi and the Sephardic. The Ashkenazi's are the main group in the US and come from Northern, Eastern and Western Europe (Russia, Germany, Poland chiefly). The Sephardim are from Southern Europe (Italy, Spain, Portugal) and North Africa for the most part. The Jews airlifted from Ethiopia in the '70s were Sephardic Jews. I bring this up because the Sephardic tradition ALLOWS the eating of rice during Passover! Right on!
So, in my own modern interpretation of Passover, while I come from an Ashkenazi background, I adopt the Sephardic traditions when celebrating this wonderful holiday. I eat rice, and I eat matzoh. And, under the principle that if wine (made with yeast) is allowed, therefore beer can't be disallowed because it is made with yeast. And if the Sephardim can eat rice, which plumps from water, that beer, therefore, should be OK for the same reason! Hooray, I'm Sephardic at least for 8 days a year.
For reinforcement, I think this is even backed up by Christian tradition, when the monks in Germany gave up bread for Lent, replacing it with Bock beer. OK, I'm stretching the point a little, but I think it is not entirely invalid.
And with that decision, I am going to drink beer during Passover!