Gas welding and stone tool: what they have in common

1. Step 1. Gas welding — uses — Acetylene. The fuel.
2. Step 2. Acetylene — uses — Chemical reaction. Calcium carbide + water.
3. Step 3. Chemical reaction — is used in — Percussion cap. Mercury fulminate explodes when struck.
4. Step 4. Percussion cap — evolved from — Flintlock. The percussion lock replaced the flintlock.
5. Step 5. Flintlock — is made of — Flint. A flint strikes the spark.
6. Step 6. Flint — is material for — Stone tool. Tools were knapped from flint and other rocks with a sharp fracture.

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