Index card and lead–acid battery: what they have in common

1. Step 1. Index card — is part of — Index card. A stiff card of standard size.
2. Step 2. Index card — is made of — Paper. Paper.
3. Step 3. Paper — is material for — Hot air balloon. It was lined with paper on the inside.
4. Step 4. Hot air balloon — uses — Archimedes’ principle. Warm air is lighter than cold — Archimedes’ principle in the atmosphere.
5. Step 5. 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.
6. Step 6. 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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