Railway semaphore and liquid-propellant rocket engine: what they have in common

1. Step 1. Railway semaphore — evolved into — Traffic light. The first traffic light was a railway-style semaphore.
2. Step 2. Traffic light — has part — Relay. Relay controllers.
3. Step 3. Relay — has part — Electromagnet. An electromagnet switches the contacts.
4. Step 4. Electromagnet — is part of — Bubble chamber. The field bends the tracks.
5. Step 5. Bubble chamber — is made of — Hydrogen. The working medium is superheated liquid hydrogen.
6. Step 6. Hydrogen — is used in — Liquid-propellant rocket engine. Hydrogen is an efficient fuel.

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