Writing is like a non-Newtonian fluid. NNFs can act like normal liquids under some circumstances, but can act like a solid in other circumstances. You can make an NNF by mixing water with corn-flour at home. If you get the mix right and hit it hard with a spoon, the mixture will become hard and the spoon will bounce off. Whereas if you stick the spoon in slowly, it will penetrate the mix easily. You can run across the surface of an NNF as the picture opposite shows. But if you stopped running, you would sink.
My writing sometimes feel like I am dealing with a non-Newtonian fluid. On some days, I fly across the surface and rattle out the pages. On other days, I go too slowly and sink. And on some days, I get the mixture wrong and just end up with a soggy mess… No writing day is a bad day, but some are definitely more productive than others.