International Masterclasses provide a unique opportunity for High school students to be "scientists for a day". 16- to 19-year-old students in 60 countries around the whole world are invited to one of about 220 nearby universities or research centers for one day in order to take part in an authentic research process. They hear lectures from active scientists and gain insight into topics and methods of basic research into the fundamentals of matter and the forces. Thus prepared, students perform measurements themselves on real data from particle physics experiment CMS at the LHC. At the end of day, up to five student groups join in a videoconference with CERN for discussion and combination of their results.