Galaxies are complicated and history dependent. Yet, recent studies have uncovered surprising correlations among the properties of galaxies. Such simplicity seems, naively, to be at odds with the paradigm of hierarchical galaxy mergers. One of the puzzling results, is the simple linear correlation between the neutral hydrogen mass and the surface area, implying that widely different galaxies share very similar neutral hydrogen surface densities. We shall see in this presentation that self-regulated star formation, driven by the competition between gravitational instabilities and mechanical feedback from supernovae, can explain the nearly constant neutral hydrogen surface density across galaxies.