Shafts for harnessing a horse and solar panel: what they have in common

1. Step 1. Shafts for harnessing a horse — is part of — Sledge. A rod.
2. Step 2. Sledge — evolved into — Cart. The wheeled cart replaced drag sledges and sleds.
3. Step 3. Cart — has part — Wheel. A cart rolls on wheels.
4. Step 4. Wheel — is part of — Lunokhod. Wheels.
5. Step 5. Lunokhod — has part — Solar panel. A solar panel.

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