Universal gravitation and pneumatic tube: what they have in common

1. Step 1. Universal gravitation — is used in — Pin tumbler lock. The pins drop into holes in the bolt under their own weight.
2. Step 2. Pin tumbler lock — is part of — Lock and key. The pins keep the cylinder from turning without a key.
3. Step 3. Lock and key — is part of — Letter box. A lockable door for collection.
4. Step 4. Letter box — uses — Postal service. The post.
5. Step 5. Postal service — includes — Pneumatic tube. The post.

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