This is the way.
When the water, in flowing down in the brooks and streams, comes to a valley from which it can not run out, it continues to run in and fill up the valley, until it reaches the level of some place where it can run out. As soon as it reaches that level, the surplus water runs out at the opening as fast as it comes in from the springs and streams, and then the lake never rises any higher.
A lake, then, is nothing but a valley full of water.
Of course, there are more valleys among mountains than any where else, and there, too, there are more streams and springs to fill them. Thus, among mountains, we generally find a great many lakes.
Outlets; feeders.
Since lakes are formed in this way, you would expect, in going around one, that you would find some streams flowing into it, and one stream flowing out. This is the case with almost all lakes. The place where the water flows out of the lake is called the outlet. The streams which flow into the lake are sometimes called the feeders. They feed the lake, as it were, with water.
Ponds without outlets.
Sometimes a lake or pond has no outlet. This is the case when there are so few streams running into it that all the water that comes can dry up from the surface of the lake, or soak away into the ground.
Sometimes you will find, among hilly pastures, a small pond, lying in a hollow, which has not any outlet, or any feeders either. Such a pond as this is fed either by secret springs beneath the ground, or else by the water which falls on the slopes around it when it is actually raining.