You have about 10–15 seconds before an audience leans in or tunes out. Don't waste your opening on "Um, hi, today I'm going to talk about…" — use a hook: a surprising fact, a bold question, a short story, a vivid image. The close matters just as much because people remember endings most — never trail off with "yeah, that's all." Land the plane: circle back to your opening (a "bookend"), or give a clear call to action. Openings and closes are the two spots worth memorizing word-for-word.