Hydrophone and knife: what they have in common

1. Step 1. Hydrophone — uses — Piezoelectric effect. A piezo element.
2. Step 2. Piezoelectric effect — is used in — Lighter. Piezo lighters.
3. Step 3. Lighter — works like — Flintlock. The same way of striking a spark.
4. Step 4. Flintlock — is made of — Flint. A flint strikes the spark.
5. Step 5. Flint — is material for — Stone tool. Tools were knapped from flint and other rocks with a sharp fracture.
6. Step 6. Stone tool — evolved into — Knife. The first knives were stone flakes.

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