When the materials of each solar system were thus set apart from the original mass of star dust or vapour, they began an independent development which led step by step, in the case of our own solar system at least, and presumably also in the case of the other suns, the fixed stars, to the formation of planets and their moons or satellites, all moving around the central sun. At this stage of the explanation the nebular hypothesis is more difficult to conceive than in the parts of it which have already been described, for we have now to understand how the planets and satellites had their matter separated from each other and from the solar centre, and why they came to revolve around that central body. These problems are best understood by noting some familiar instances connected with the movement of fluids and gases toward a centre. First let us take the case of a basin in which the water is allowed to flow out through a hole in its centre. When we lift the stopper the fluid for a moment falls straight down through the opening. Very quickly, however, all the particles of the water start to move toward the centre, and almost at once the mass begins to whirl round with such speed that, although it is working toward the middle, it is by its movement pushed away from the centre and forms a conical depression. As often as we try the experiment, the effect is always the same. We thus see that there is some principle which makes particles of fluid that tend toward a centre fail directly to attain it, but win their way thereto in a devious, spinning movement.