The gas-phase oxygen abundance, i.e. metallicity, of a galaxy is set by the interplay between star formation and gas flows. Metals are dispersed into the interstellar medium by stellar winds and supernovae. Metals accumulate in the interstellar medium of star-forming galaxies and provide a record of star-formation. However, inflows of unenriched gas into galaxies and metal-rich outflows of gas from galaxies can both reduce the metallicity. Thus, measurements of the metallicity across cosmic time provide important constraints for understanding the cycling of gas through galaxies as they build their stellar mass and evolve. We have measured the chemical evolution of galaxies over the last 10 billion years of cosmic time. These measurements provide a coherent picture of how galaxies enrich as they build their stellar mass. We show that the chemical evolution of star-forming galaxies is very simple. The metallicity of star-forming galaxies at z < 1.5 only depends on the stellar-to-gas mass ratio. The relation between metallicity and stellar-to-gas mass ratio is an universal relation followed by all galaxies as they evolve.