Lens replacement and fuel cell: what they have in common

1. Step 1. Lens replacement — uses — Ultrasound. Phacoemulsification.
2. Step 2. Ultrasound — is used in — Sonar. Ultrasonic pulses.
3. Step 3. Sonar — is part of — Submarine. Underwater acoustics.
4. Step 4. Submarine — has part — Lead–acid battery. Underwater, electric motors run on batteries.
5. Step 5. Lead–acid battery — uses — Electrochemical reaction. A reversible reaction of lead and lead dioxide in sulfuric acid.
6. Step 6. Electrochemical reaction — is used in — Fuel cell. The reaction runs electrochemically, without combustion.

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