I ran across this chraming poem by Butler Brannan in an old magazine on Google Books. Oringinally published in 1901, it compares creativity to a factory. The brilliant bit is that it acknowledges that we don't always control what the final product of that factory will be.  I think we've all had days when our heads produced nothing but hair.

My head is like a factory,
the windows are my eyes;
The Furnace is my mouth, – you see
I feed it meats or pies.

And when its Hunger I appease
My Head will do its share,
Sometimes producing Rhymes like these
And sometimes only Hair.