Zip and watt steam engine: what they have in common

1. Step 1. Zip — is part of — Jeans. A zip.
2. Step 2. Jeans — uses — Indigo. Indigo.
3. Step 3. Indigo — evolved into — Synthetic dye. Synthesizing natural dyes.
4. Step 4. Synthetic dye — is made of — Coal. Aniline from coal tar.
5. Step 5. Coal — is used in — Watt steam engine. Steam engines were fired with coal.

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