Skate boot and tunnelling shield: what they have in common

1. Step 1. Skate boot — is part of — Ice skates. A boot.
2. Step 2. Ice skates — evolved into — Roller skates. Skates for dry land.
3. Step 3. Roller skates — has part — Ball bearing. Bearings.
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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