Replaceable blade and conveyor belt: what they have in common

1. Step 1. Replaceable blade — is part of — Scalpel. A disposable blade.
2. Step 2. Scalpel — is made of — Stainless steel. Stainless steel.
3. Step 3. Stainless steel — is material for — Fountain pen. The nib of a modern fountain pen is stainless steel.
4. Step 4. Fountain pen — evolved into — Ballpoint pen. A replacement for the fountain pen.
5. Step 5. Ballpoint pen — has part — Ball bearing. A ball in a socket — essentially a ball bearing.
6. Step 6. Ball bearing — is part of — Conveyor belt. The rollers turn on bearings.

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