The hook is Barbry Allen herself. You learn in the first three lines what a knockout she must be.

The notion of a hook comes from newspaper writing where the challenge is to open the story with language that will induce the reader to read the rest of the story in spite of the difficulties (small print, column layyout, page-folding). A good hook is usually found in the first paragraph and is language used to capture the interest.

In songs, the hook is the language (or music, i guess) used to capture the intereest of the listener.

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