Email and conveyor belt: what they have in common

1. Step 1. Email — evolved from — Postal service. Electronic mail.
2. Step 2. Postal service — uses — Steam locomotive. Mail trains.
3. Step 3. Steam locomotive — evolved into — Diesel locomotive. It replaced the steam locomotive.
4. Step 4. Diesel locomotive — has part — Bogie. Bogies with wheelsets.
5. Step 5. Bogie — has part — Ball bearing. Axle boxes with roller bearings.
6. Step 6. Ball bearing — is part of — Conveyor belt. The rollers turn on bearings.

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