Hammerstone and railway semaphore: what they have in common

1. Step 1. Hammerstone — is used in — Stone tool. A hammerstone for knapping.
2. Step 2. Stone tool — evolved into — Hammer. The first hammer was a stone in the hand.
3. Step 3. Hammer — uses — Lever. The handle lengthens the arm of the blow.
4. Step 4. Lever — is used in — Railway semaphore. The semaphore is set by a lever.

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