Reaction force and modem: what they have in common

1. Step 1. Reaction force — is used in — Rocket. A rocket moves by reaction force.
2. Step 2. Rocket — evolved into — Guided missile. A rocket with guidance.
3. Step 3. Guided missile — has part — Infrared seeker. A seeker.
4. Step 4. Infrared seeker — has part — Photodiode. A photodetector.
5. Step 5. Photodiode — is used in — Fax. Scanning with a photocell.
6. Step 6. Fax — has part — Modem. It transmits an image over a telephone line.

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