To form a cloud droplet, water needs a seed to condense on it. Without seed, no cloud! Two kinds of seed, named primary and secondary aerosol, can be found in the atmosphere. Primary aerosols are injected directly into the atmosphere, like for example Sahara dust or volcanoes hash. Secondary aerosols are formed from the agglomeration of gas molecules into a particle. At CERN, in the CLOUD experiment we investigate the formation of secondary aerosols. The effect of galactic cosmic rays on secondary aerosol formation is investigated using the CERN beam from the Proton Synchrotron. In this presentation, I will introduce the CLOUD experiment, our latest results and a comparison with state of the art ambient data.