Letter box and mechanical clock: what they have in common

1. Step 1. Letter box — has part — Lock and key. A lockable door for collection.
2. Step 2. Lock and key — has part — Pin tumbler lock. The pins keep the cylinder from turning without a key.
3. Step 3. Pin tumbler lock — uses — Universal gravitation. The pins drop into holes in the bolt under their own weight.
4. Step 4. Universal gravitation — is used in — Escapement. The pace of the clock is set by a falling weight.
5. Step 5. Escapement — is part of — Mechanical clock. The escapement releases energy in portions and sets the rhythm.

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