Compressed air and stone tool: what they have in common

1. Step 1. Compressed air — is used in — Supercharger. It forces air into the cylinders.
2. Step 2. Supercharger — has part — Belt drive. Belt-driven from the crankshaft.
3. Step 3. Belt drive — is part of — Combine harvester. Belt drives.
4. Step 4. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.
5. Step 5. Sickle — evolved from — Stone tool. The first sickles had flint inserts.

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