Here's how it works:

* Rain Gauge: A rain gauge is a device used to collect and measure rainfall. It typically has a funnel that directs rainwater into a graduated cylinder.

* Measurement: The amount of rainfall is measured as the height of the water collected in the cylinder.

* Units: Since the height is measured in millimeters, the rainfall is also expressed in millimeters.

Example: If a rain gauge collects 10 millimeters of rainwater, then the rainfall is said to be 10 mm.