Ballpoint pen and chisel: what they have in common

1. Step 1. Ballpoint pen — has part — Ball bearing. A ball in a socket — essentially a ball bearing.
2. Step 2. Ball bearing — is part of — Conveyor belt. The rollers turn on bearings.
3. Step 3. Conveyor belt — is part of — Tunnel boring machine. Spoil is removed by a conveyor.
4. Step 4. Tunnel boring machine — evolved from — Tunnelling shield. A development of the tunnelling shield.
5. Step 5. Tunnelling shield — is used in — Tunnel. Shield tunnelling.
6. Step 6. Tunnel — uses — Chisel. Hezekiah’s Tunnel was cut by hand with chisels.

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