You should hammer your iron when it is glowing hot.

Publilius Syrus was a 1st‑century BCE writer of Latin mimes whose lines survived chiefly as aphorisms (the Sententiae) excerpted and copied for rhetorical and moral instruction. Many of these maxims circulated in schools and commonplace books in antiquity and the Middle Ages, often detached from their original dramatic settings. The image of the smith hammering heated iron belongs to a long Greco‑Roman tradition of craft metaphors used to teach practical wisdom: act at the moment when conditions are most favorable. In later European reception, Syrus’s sententiae were frequently translated and paraphrased, which helps explain why this saying is commonly encountered in English in several slightly different forms.

The maxim urges timely action: opportunities have a window in which effort is most effective, just as iron can be shaped only when it is hot. It implies that preparation and decisiveness matter as much as desire—one must recognize the “glowing” moment and apply force then, rather than waiting until circumstances cool and become resistant. The metaphor also carries a moral edge typical of Syrus’s sententiae: procrastination is not neutral but a failure of judgment, because delay can harden problems and waste favorable conditions. In modern terms, it counsels seizing momentum—whether in work, persuasion, reform, or personal change—while the conditions that make success possible are present.

“Strike while the iron is hot.”
“Strike the iron while it is hot.”
“While the iron is hot, strike.”