Ladder rungs and watt steam engine: what they have in common

1. Step 1. Ladder rungs — is part of — Ladder. Rungs for the feet.
2. Step 2. Ladder — is part of — Fire truck. An extending ladder.
3. Step 3. Fire truck — evolved from — Fire pump. A pump on a motor-vehicle chassis.
4. Step 4. Fire pump — has part — Piston pump. A fire pump is a piston pump.
5. Step 5. Piston pump — evolved into — Watt steam engine. The first steam engines (Newcomen’s) pumped water out of mines by driving pumps.

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