Wall heating flues and smartphone: what they have in common

1. Step 1. Wall heating flues — is part of — Hypocaust. Pipes in the walls for hot air.
2. Step 2. Hypocaust — uses — Combustion. Hot air from the furnace heats the floor.
3. Step 3. Combustion — is used in — Gas lamp. The flame gives the light.
4. Step 4. Gas lamp — evolved into — Street light. Gas lamps.
5. Step 5. Street light — has part — LED. LEDs.
6. Step 6. LED — is part of — Smartphone. The camera flash.

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