Write the thought down. Vague ideas can’t survive a full sentence.

A thought in your head stays impressive as long as it stays blurry. Writing it as a full sentence forces the decisions you have been avoiding, about what exactly you mean and what follows from it, and some ideas turn out to have nothing in the middle. That is a cheap thing to discover on paper.

Write down the plan or the argument you have been carrying around, in sentences rather than fragments. Wherever it falls apart is the part worth working on. And if it survives the page, you now have something you can say to somebody.