Trigonometry and electric fan: what they have in common

1. Step 1. Trigonometry — is used in — Sextant. Navigational calculations.
2. Step 2. Sextant — has part — Mirror. Mirrors — the principle of double reflection.
3. Step 3. Mirror — is part of — Periscope. Mirrors or prisms.
4. Step 4. Periscope — is part of — Submarine. A periscope.
5. Step 5. Submarine — has part — Propeller. A propeller.
6. Step 6. Propeller — is part of — Electric fan. The blades.

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