Letter slot and knife: what they have in common

1. Step 1. Letter slot — is part of — Letter box. A slot protected from rain.
2. Step 2. Letter box — uses — Postal service. The post.
3. Step 3. Postal service — uses — Steam locomotive. Mail trains.
4. Step 4. Steam locomotive — has part — Crank. Connecting rods and cranks carry the piston stroke to the wheels.
5. Step 5. Crank — is part of — Meat grinder. A handle.
6. Step 6. Meat grinder — has part — Knife. A cross-shaped blade.

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