Spear-thrower shaft and boiler: what they have in common

1. Step 1. Spear-thrower shaft — is part of — Spear-thrower. A lever that extends the arm.
2. Step 2. Spear-thrower — uses — Spear. It throws darts.
3. Step 3. Spear — has part — Stone tool. Early spearheads were made of stone.
4. Step 4. Stone tool — evolved into — Hammer. The first hammer was a stone in the hand.
5. Step 5. Hammer — evolved into — Steam hammer. A steam hammer is a hammer lifted by steam, used to forge huge parts.
6. Step 6. Steam hammer — has part — Boiler. A boiler supplies the steam.

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