Digital thermometer and railway track: what they have in common

1. Step 1. Digital thermometer — has part — Liquid-crystal display. The reading is on an LCD.
2. Step 2. Liquid-crystal display — has part — LED. LED backlighting.
3. Step 3. LED — is part of — Street light. LEDs.
4. Step 4. Street light — is part of — Tunnel. Lighting.
5. Step 5. Tunnel — is used in — Metro. Tunnels.
6. Step 6. Metro — uses — Railway track. It runs on rails.

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