Rain gauge and lead–acid battery: what they have in common

1. Step 1. Rain gauge — is part of — Weather station. A rain gauge.
2. Step 2. Weather station — has part — Thermometer. A thermometer.
3. Step 3. Thermometer — is made of — Mercury. Mercury thermometers.
4. Step 4. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
5. Step 5. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.

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