Water tower and petri dish: what they have in common

1. Step 1. Water tower — is part of — Plumbing. Maintaining pressure.
2. Step 2. Plumbing — works like — Aqueduct. An aqueduct brings water to a city, plumbing distributes it inside; Minoan pipes are older than the Roman aqueducts.
3. Step 3. Aqueduct — works like — Sewerage. Both systems carry water by gravity through channels: one drains waste, the other supplies water; the drains of Mohenjo-daro are older than the Roman aqueducts.
4. Step 4. Sewerage — has part — Sewage treatment plant. Treatment plants.
5. Step 5. Sewage treatment plant — uses — Culture medium (agar). Waste water is cleaned by activated sludge — bacteria.
6. Step 6. Culture medium (agar) — uses — Petri dish. Agar in dishes.

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