Inflatable catheter balloon and railway track: what they have in common

1. Step 1. Inflatable catheter balloon — is part of — Catheter. It inflates and holds the catheter in place.
2. Step 2. Catheter — is part of — IV drip. A catheter.
3. Step 3. IV drip — uses — Gravity flow. The solution flows under gravity.
4. Step 4. Gravity flow — is used in — Water wheel. Falling or flowing water turns the wheel.
5. Step 5. Water wheel — is used in — Rolling mill. The first mills were turned by water.
6. Step 6. Rolling mill — is used in — Railway track. Rails are rolled.

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