Imagine a herd a buffalo, moving across the plains.
The predators are there as well, and they attack the buffalo. They feast easily on the older, slower buffalo at the back of the pack, but the herd as a whole moves on and thrives.
Now we all know that alcohol kills brain cells, but it's the older, slower brain cells at the back of the pack that get it first. Your brain, as a whole, needs those to be killed off in order to thrive.
My current brain cell killer of choice: