Petri dish and electrical telegraph: what they have in common

1. Step 1. Petri dish — is used in — Culture medium (agar). Agar in dishes.
2. Step 2. Culture medium (agar) — is used in — Sewage treatment plant. Waste water is cleaned by activated sludge — bacteria.
3. Step 3. Sewage treatment plant — is made of — Concrete. The tanks are concrete.
4. Step 4. Concrete — is material for — Maglev. The guideway is concrete.
5. Step 5. Maglev — uses — Electromagnetism. Electromagnets hold and move the train.
6. Step 6. Electromagnetism — is used in — Electrical telegraph. A telegraph receiver is an electromagnet that attracts an armature.

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