Turn on the tap. The water starts flowing out. At the top the stream of water is the size of the tap opening but it grows smaller as it falls.
What's the reason?
Surface tension tends to hold the water in a solid stream at the top but as it falls its speed increases because of the pull of gravity. Since it is a continuous flow the amount passing any point must be the same. The amount of flow is the cross sectional area of the stream multiplied by the speed of flow of the water. So as the water is flowing faster as it falls, the stream narrows.