When water flows down a river, the current is measured by the volume of water that passes by a cross-section of the river divided by the time taken for the water to pass. For example, if the current of a river is 2,000 m³/min (cubic meters per minute), this means that in every minute 2,000 cubic meters pass by every part of the river. We can write this as (2,000 m²) X (1 m/min). If the river narrows, the 2,000 cubic meters of water still must pass in one minute because the water from behind continues to flow downriver. Since the river is narrower, the area (measured in meters squared, m²) is smaller, so the speed (measured in meters per minute) must increase. This fact is called the principle of continuity.

As in all of physics, objects move as a result of a net force. In a fluid the net force comes about because there is a difference in pressure between two points. The fluid flows in the direction of decreasing pressure.