Dark room box and elevator: what they have in common

1. Step 1. Dark room box — is part of — Camera obscura. A darkened room or box.
2. Step 2. Camera obscura — evolved into — Camera. A camera is a camera obscura with a light-sensitive layer.
3. Step 3. Camera — has part — Shutter. A shutter.
4. Step 4. Shutter — has part — Spring. A spring mechanism.
5. Step 5. Spring — is part of — Elevator safety brake. Springs throw out the catches when the rope goes slack.
6. Step 6. Elevator safety brake — is part of — Elevator. Otis’s safety brakes.

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