Nail and car: what they have in common

1. Step 1. Nail — uses — Wedge. The point of a nail pushes the wood apart.
2. Step 2. Wedge — is used in — Plough. The ploughshare parts the soil like a wedge.
3. Step 3. Plough — evolved into — Steel plough. A steel share.
4. Step 4. Steel plough — is used in — Tractor. Ploughing with a tractor.
5. Step 5. Tractor — has part — Transmission (gearbox). A transmission.
6. Step 6. Transmission (gearbox) — is part of — Car. The gearbox matches engine speed to the wheels.

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