From pipes that lead to a well underground or to a tank of rainwater. Let us start thinking about the oceans. We know that if the sun hits the water, some of it will get hot enough to turn into a gas, water vapor. The when this gas rises it turns into water droplets in a cloud. As the droplets get bigger they start falling as rain.
Lets try and find out.
So let us look at the hot water that is salty inside the plastic bag. Compare it to a drop of the salty water. Hey it isn't salty so the rainwater than came from the evaporated ocean water is not salty. But how did the ocean water get salty in the first place. Well if there is salt in the dirt or rocks what will happen as the rainwater goes through.