Chin strap and lead–acid battery: what they have in common

1. Step 1. Chin strap — is part of — Helmet. A mount.
2. Step 2. Helmet — is made of — Leather. Helmet liners and simple helmets were made of leather.
3. Step 3. Leather — is material for — Submarine. It was covered with leather.
4. Step 4. Submarine — has part — Lead–acid battery. Underwater, electric motors run on batteries.

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