Surfboard and modem: what they have in common

1. Step 1. Surfboard — uses — Archimedes’ principle. Buoyancy.
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 — uses — Electrochemical reaction. A reversible reaction of lead and lead dioxide in sulfuric acid.
5. Step 5. Electrochemical reaction — is used in — Fax. The paper darkened under current.
6. Step 6. Fax — has part — Modem. It transmits an image over a telephone line.

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