Rubber shoe sole and liquid-propellant rocket engine: what they have in common

1. Step 1. Rubber shoe sole — is part of — Trainers. A rubber outsole.
2. Step 2. Trainers — is made of — Vulcanized rubber. The sole.
3. Step 3. Vulcanized rubber — is material for — Weather balloon. The envelope is rubber or latex.
4. Step 4. Weather balloon — is made of — Hydrogen. Filled with hydrogen or helium.
5. Step 5. Hydrogen — is used in — Liquid-propellant rocket engine. Hydrogen is an efficient fuel.

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