Trowel and optical telegraph: what they have in common

1. Step 1. Trowel — is used in — Brick. Bricklaying with a trowel.
2. Step 2. Brick — is material for — Chemical plant. The furnaces and towers are brick.
3. Step 3. Chemical plant — has part — Programmable logic controller. Process control automation.
4. Step 4. Programmable logic controller — evolved from — Relay. It replaced relay cabinets.
5. Step 5. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.
6. Step 6. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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