I like this one because it sounds plausible..
 
So after all these years of trying to figure this out.....

Proven...

A herd of buffalo can move only as fast as the slowest buffalo.  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 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.