Ladder side rails and aeroplane: what they have in common

1. Step 1. Ladder side rails — is part of — Ladder. Two lengthwise uprights.
2. Step 2. Ladder — uses — Inclined plane. Climbing to a height.
3. Step 3. Inclined plane — evolved into — Screw. A screw is an inclined plane wrapped around a cylinder.
4. Step 4. Screw — is used in — Propeller. A propeller “screws” itself into the water, like an Archimedes’ screw.
5. Step 5. Propeller — is part of — Aeroplane. Propeller aircraft.

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