Mobile app and stone tool: what they have in common

1. Step 1. Mobile app — is used in — Bike sharing. An app.
2. Step 2. Bike sharing — uses — Electric bicycle. Electric bicycles.
3. Step 3. Electric bicycle — has part — Roller chain. The chain drive from the pedals remains.
4. Step 4. Roller chain — is part of — Combine harvester. Chain drives power the working parts.
5. Step 5. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.
6. Step 6. Sickle — evolved from — Stone tool. The first sickles had flint inserts.

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