When you know not what to write!

You say you haven't written for a while,
you take a pen and paper.
This time you begin with the story of the Nile;
But then you switch to vapour!
Again you stop, and move to the window
to write about its panes.
But you feel so odd, and shift to Lindow;
and start describing its lanes.
Again the paper is thrown to the bin;
crumbled like a pie.
Then you leave the chair with a grin,
to let your craving pass by.