Master photographer: Why does the longer the shutter time, the greater the light input?

For example, if you use a water pipe to drain water, will the water output be the same for one minute and two minutes? The same is true for camera exposure. When the light is constant and the aperture size is fixed, the longer the shutter is opened, the more light will enter the lens. If you understand the exposure, that is, how much light you get per unit time, it is not difficult to understand.