Imagine a cylinder that is full of water. A straw is attached to the bottom of the cylinder so that the water can slowly pour out.

Now imagine that the water is pouring out the bottom of the cylinder and you are looking at it from above.

What shape would the surface of the water take on when the cylinder has is still full?

What shape would the surface of the water take on when the cylinder is three quarters full?

What shape would the surface of the water take on when the cylinder is half full?

What shape would the surface of the water take on when the cylinder is a quarter full?

The answer is that the surface of the water would take on the shape of a circle in all of these situations.