Shovel and tank: what they have in common

1. Step 1. Shovel — has part — Handle. A handle.
2. Step 2. Handle — is part of — Saw. A handle.
3. Step 3. Saw — evolved into — Chainsaw. A mechanized saw.
4. Step 4. Chainsaw — has part — Sprocket. A sprocket drives the chain.
5. Step 5. Sprocket — is part of — Continuous track. The drive sprocket engages the track and pulls it round — just as a bicycle sprocket pulls the chain.
6. Step 6. Continuous track — is part of — Tank. Tracks let a tank cross trenches and rough ground.

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