Gas mask and petri dish: what they have in common

1. Step 1. Gas mask — evolved from — Respirator. Respiratory protection.
2. Step 2. Respirator — evolved from — Surgical mask. A filtering mask.
3. Step 3. Surgical mask — uses — Germ theory of disease. Protection against droplets.
4. Step 4. Germ theory of disease — is used in — Petri dish. Growing bacterial cultures.

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