I have not seen anyone explain this as well as Cliff Clavin, on
  Cheers. One afternoon at Cheers, Cliff Clavin was explaining the
  Buffalo Theory to his buddy Norm. Here's how it went: "Well ya see,
  Norm, it's like this... A herd of buffalo can only move as fast as
  the slowest buffalo. And when the herd is hunted, it is the slowest
  and weakest ones at the back that are killed first. This natural
  selection is good for the herd as a whole, because the general speed
  and health of the whole group keeps improving by the regular killing
  of the weakest members. "In much the same way, the human brain can
  only operate as fast as the slowest brain cells. Excessive intake of
  alcohol, as we know, kills brain cells. But naturally, it attacks
  the slowest and weakest brain cells first. In this way, regular
  consumption of beer eliminates the weaker brain cells, making the
  brain a faster and more efficient machine. That's why you always
  feel smarter after a few beers."

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