Padlock and steam locomotive: what they have in common

1. Step 1. Padlock — is a kind of — Lock and key. A padlock.
2. Step 2. Lock and key — is part of — Letter box. A lockable door for collection.
3. Step 3. Letter box — uses — Postal service. The post.
4. Step 4. Postal service — uses — Steam locomotive. Mail trains.

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