Pinwheel with sliding teeth and steam locomotive: what they have in common

1. Step 1. Pinwheel with sliding teeth — is part of — Arithmometer. A wheel with retractable teeth sets a digit.
2. Step 2. Arithmometer — has part — Crank. The handle for adding and subtracting.
3. Step 3. Crank — is part of — Steam locomotive. Connecting rods and cranks carry the piston stroke to the wheels.

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