Spindle whorl and steam locomotive: what they have in common

1. Step 1. Spindle whorl — is part of — Spindle. A flywheel weight for spinning.
2. Step 2. Spindle — evolved into — Spinning wheel. A wheel turns the spindle.
3. Step 3. Spinning wheel — has part — Crank. A foot drive through a crank.
4. Step 4. Crank — is part of — Steam locomotive. Connecting rods and cranks carry the piston stroke to the wheels.

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