Gold and tunnelling shield: what they have in common

1. Step 1. Gold — is material for — Fountain pen. Expensive nibs are gold: it does not corrode and flexes softly.
2. Step 2. Fountain pen — evolved into — Ballpoint pen. A replacement for the fountain pen.
3. Step 3. Ballpoint pen — has part — Ball bearing. A ball in a socket — essentially a ball bearing.
4. Step 4. Ball bearing — is part of — Conveyor belt. The rollers turn on bearings.
5. Step 5. Conveyor belt — is part of — Tunnel boring machine. Spoil is removed by a conveyor.
6. Step 6. Tunnel boring machine — evolved from — Tunnelling shield. A development of the tunnelling shield.

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