Lock and key and paper machine: what they have in common

1. Step 1. Lock and key — is part of — Letter box. A lockable door for collection.
2. Step 2. Letter box — uses — Postal service. The post.
3. Step 3. Postal service — is used in — Postage stamp. Paying for delivery.
4. Step 4. Postage stamp — is made of — Paper. Paper.
5. Step 5. Paper — uses — Paper machine. Industrial production.

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