Water level markings and gas lamp: what they have in common

1. Step 1. Water level markings — is part of — Water clock (clepsydra). Hour marks on the wall.
2. Step 2. Water clock (clepsydra) — uses — Gravity flow. Water flows out at a constant rate under gravity.
3. Step 3. Gravity flow — is used in — Flush toilet. Flushing by gravity.
4. Step 4. Flush toilet — has part — Valve. The cistern’s float valve.
5. Step 5. Valve — is part of — Gas lamp. A gas supply valve.

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