Ferry and electrical grid: what they have in common

1. Step 1. Ferry — uses — Archimedes’ principle. It stays afloat by Archimedes’ principle.
2. Step 2. Archimedes’ principle — is used in — Hydrometer. A hydrometer floats: the buoyant force depends on the liquid's density, so how deep it sinks shows the density.
3. Step 3. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
4. Step 4. Lead–acid battery — is part of — Uninterruptible power supply. A battery.
5. Step 5. Uninterruptible power supply — is part of — Data centre. Backup power.
6. Step 6. Data centre — uses — Electrical grid. Power.

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