Fine sand and steam locomotive: what they have in common

1. Step 1. Fine sand — is material for — Hourglass. Fine, uniform sand.
2. Step 2. Hourglass — uses — Gravity flow. Sand pours through under gravity.
3. Step 3. Gravity flow — is used in — Sewerage. Waste water flows down a gradient.
4. Step 4. Sewerage — is made of — Brick. The drains of Mohenjo-daro are lined with fired brick.
5. Step 5. Brick — is material for — Chimney. Chimneys are brick.
6. Step 6. Chimney — is part of — Steam locomotive. Smoke from the firebox leaves through the locomotive's chimney.

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