Semaphore arm and plough: what they have in common

1. Step 1. Semaphore arm — is part of — Railway semaphore. The position of the arm is the signal.
2. Step 2. Railway semaphore — uses — Lever. The semaphore is set by a lever.
3. Step 3. Lever — is used in — Hoe. The handle works as a lever when loosening soil.
4. Step 4. Hoe — evolved into — Plough. The plough mechanized loosening the soil with a hoe.

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