Touch hole and micrometer: what they have in common

1. Step 1. Touch hole — is part of — Hand cannon. Igniting the charge.
2. Step 2. Hand cannon — evolved into — Musket. A hand-held gun.
3. Step 3. Musket — has part — Trigger. A trigger.
4. Step 4. Trigger — is part of — Revolver. Pulling the trigger turns the cylinder and drops the hammer.
5. Step 5. Revolver — uses — Interchangeable parts. Colt used machine production of interchangeable parts.
6. Step 6. Interchangeable parts — uses — Micrometer. Precise measurement ensures interchangeability.

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