Gas around galaxies, the circumgalactic medium, contains signatures of galactic outflows and accretion, which are crucial processes driving the evolution of galaxies. To better understand these processes, we investigate the chemical evolution of the cool circumgalactic medium spanning the redshift range 0.4 < z < 2.5, probed via the largest metal absorber catalog from the Sloan Digital Sky Survey. Utilizing the dataset, we not only measure the abundances of more than 10 metal elements but also constrain the physical conditions of the circumgalactic gas. In this talk, I will show that (1) the metallicity of the circumgalactic gas evolves consistently with the metal production of the Universe and reaches the solar value at redshift ~1; (2) the metals are carried by small dense clouds with the sizes ~10 pc. These findings uncover a new picture: galaxies are surrounded by ~10^6 metal-enriched gas clouds---a non-negligible amount revealed for the first time.