Retractable roof and laser: what they have in common

1. Step 1. Retractable roof — is part of — Stadium. A retractable roof.
2. Step 2. Stadium — has part — Street light. Floodlights.
3. Step 3. Street light — has part — Photoresistor. Switching on at dusk.
4. Step 4. Photoresistor — is made of — Semiconductor. Selenium and cadmium sulfide are semiconductors.
5. Step 5. Semiconductor — is material for — Laser. Maiman’s first laser was a ruby crystal; today it is more often a semiconductor.

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