What are cosmic particles and where do they come from? These are questions which are not only fascinating for scientists. With the CosMO experiment high-school students can perform their own hands-on experiments and become familiar with modern scientific working methods. The detector consists of three scintillation counter boxes. An electronic board, developed by Fermilab for the QuarkNet Project, realizes the trigger condition, the data acquisition and the GPS data taking. With a Python program running on a netbook under Linux, the trigger and data taking conditions can be defined. The program also manages the data storage and the online display of particle rates. All components fit into a single notebook bag. The detector can also be operated with a small 5 Volt battery pack, independently from the power grid. Possible student experiments comprise, for instance, the measurement of cosmic particle rates in dependence on the zenith angle, particle showers in dependence on the detector distance, and the lifetime of muons. Twenty detectors have been built by DESY. They are used within the German outreach network "Netzwerk Teilchenwelt" by 15 astroparticle-research institutes for practical project works. The network program also includes workshops for teachers and the mentoring of the experiments performed by school students.