Metal sphere and sickle: what they have in common

1. Step 1. Metal sphere — is part of — Van de Graaff generator. It stores charge.
2. Step 2. Van de Graaff generator — is made of — Vulcanized rubber. The belt is rubber.
3. Step 3. Vulcanized rubber — is material for — Rover safety bicycle. The tyres are rubber.
4. Step 4. Rover safety bicycle — has part — Roller chain. A chain drive.
5. Step 5. Roller chain — is part of — Combine harvester. Chain drives power the working parts.
6. Step 6. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.

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