Put a lid on it

Some of us are old enough to remember the method for making popcorn in the days before microwave ovens. We put the kernals of corn and some Wesson oil into a "popcorn popper" - a large bowl with an electrical cord attached to it. Then we'd plug it in, wait for a few minutes while the corn was popping, and, before long, we had a tasty treat. Except we've forgotten one important step.
We have to put the lid on the popcorn popper or the popcorn will fly all over the place. There's no control without the lid.
The same principle applies to writing. Without some central theme, our thoughts will fly all over the place, just like the popcorn does without a lid.
Every sentence in a paragraph should have a direct relationship to the first sentence. Every paragraph should have a direct relationship to the theme of whatever we're writing about. Otherwise, we have literary chaos.
The lesson is simple. Keep a lid on it.