The insulin story

And Lo, our body came together in a hodge podge way
Our brain cells are primitive, and must be coddled, the bloodstream that
bathes them constantly with oxygen and glucose, it's sole energy source,
mus always be there or we faint.  We shut off immediately.

The bloodstream is shared by the muscles,
and when we get scared because a Cougar is chasing us,
well, those muscle cells could very easily suck up all the glucose out
of the bloodstream, to use for the running away process.
But if they did that, the brain would shut down, we would faint,
the Cougar would grab us by the ass and take us back to her apartment...
but that is another story.

So, our body has evolved to have a butler at the cell walls of the muscle cells,
and they wont let in any sugar, unless a special signal chemical  is present in the bloodstream
to ring the bell.  Then the butler will open up and let in some glucose.

The special signal chemical is Insulin.  

That way, sugar glucose is always available in the blood for use by the brain,
and the muscle cells will just have to wait in line.