Cosmic rays are particles that are constantly bombarding the Earth. We do not know where they come from, but we know that when they reach the atmosphere they produce a large number of particles. Some of them can be detected. Some of them are muons. But, what are muons? How can we detect them? What can we learn about cosmic rays from them? What else can we use a muon detector for?
Come and build your own muon detector and learn how particles interact with matter, how they are created and destroyed in the atmosphere, what is a particle shower and how particle detectors work. Be an experimental physicist: design your setup, answer your questions, ask new ones.