I will show there is a new fundamental relation for typical disk galaxies between stellar surface density, gas surface density and local gas-phase metallicity. The finding was made possible by recent IFU surveys (e.g., PINGS, CALIFA, MaNGA). I will show the observations by the latest SDSS-IV/MaNGA survey and demonstrate that the three parameters form a tight relationship. I will introduce a simple local leaky-box model and show that the tight relation is a fundamental prediction of the model. I will discuss the implications of this new development for the cosmological evolution of disk galaxies like our own Milky Way.