Film projector and sewage treatment plant: what they have in common

1. Step 1. Film projector — uses — Photographic film. It shows film.
2. Step 2. Photographic film — is made of — Celluloid. The first flexible film was celluloid.
3. Step 3. Celluloid — works like — Nail polish. Both are nitrocellulose: celluloid is a solid plastic, the polish is a solution of it that dries into a film.
4. Step 4. Nail polish — uses — Dye. Pigments and dyes give the polish its colour.
5. Step 5. Dye — uses — Fermentation. Some dyes (indigo) were obtained by fermenting plants.
6. Step 6. Fermentation — is used in — Sewage treatment plant. Activated sludge is microbes.

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