Glass test tube and submarine: what they have in common

1. Step 1. Glass test tube — is part of — Test tube. A glass tube with a rounded bottom.
2. Step 2. Test tube — is made of — Glass. Glass.
3. Step 3. Glass — is material for — Hydrometer. A glass tube with a weight at the bottom and a scale at the top.
4. Step 4. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
5. Step 5. Lead–acid battery — is part of — Submarine. Underwater, electric motors run on batteries.

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