An educational activity, the International Particle Physics Masterclasses, was developed by the International Particle Physics Outreach Group with the aim to bring the excitement of cutting-edge particle-physics research into the classroom. Thousands of pupils, every year since 2005, in many countries all over the world, are hosted in research centres or universities close to their schools and become "scientists for a day" as they are introduced to the mysteries of particle physics. In 2012, 10 000 students from 148 institutions in 31 countries participated in this popular event over a month period. The program of a typical day includes lectures that give insight to topics and methods of fundamental research followed by a "hands-on" session where the high-school students perform themselves measurements on real data from particle-physics experiments. The last two years data from the ALICE, ATLAS and CMS experiments at LHC were used. The performed measurements and the employed methodology are presented.