Nobody reads a caption. People read a first line, and then decide whether the caption deserves them. That decision takes under a second, and most brands lose it before it starts.
The classic mistake is writing the post first and the opener last, as a summary. A summary is the one thing a hook must never be: if the first line tells the whole story, there is no reason to tap more.
Write ten, keep one
For every post that matters, draft ten first lines before you write anything else. Aim for tension, a specific number, or a quiet confession.