Blackboard and mechanical clock: what they have in common

1. Step 1. Blackboard — uses — Chalk. People write with chalk.
2. Step 2. Chalk — is a kind of — Lime. Chalk is calcium carbonate, like limestone.
3. Step 3. Lime — is material for — Cistern. The walls were coated with lime plaster against leaks.
4. Step 4. Cistern — is made of — Stone. Cisterns were cut into rock or lined with stone.
5. Step 5. Stone — is material for — Water clock (clepsydra). The clepsydra from Karnak is a stone vessel.
6. Step 6. Water clock (clepsydra) — evolved into — Mechanical clock. Mechanical clocks replaced water clocks, swapping the flow of water for an escapement.

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