Hook spur and personal computer: what they have in common

1. Step 1. Hook spur — is part of — Spear-thrower. A hook against which the dart rests.
2. Step 2. Spear-thrower — uses — Lever. It extends the arm — a lever.
3. Step 3. Lever — is part of — Typewriter. Each key is a system of levers.
4. Step 4. Typewriter — has part — Keyboard. The keys drive levers with type.
5. Step 5. Keyboard — is part of — Personal computer. A keyboard.

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