Submarine and LIGO: what they have in common

1. Step 1. Submarine — has part — Periscope. A periscope.
2. Step 2. Periscope — has part — Mirror. Mirrors or prisms.
3. Step 3. Mirror — is part of — Laser. A resonator made of mirrors.
4. Step 4. Laser — is used in — LIGO. A laser.

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