According to our present knowledge, the world is made of two elementary particle types, quarks and leptons. “Elementary” here means that they are not made of any other, smaller particles themselves. Hence, they are the fundamental building blocks in Nature. They can interact via the exchange of bosons, resulting in attractive or repulsive forces and consequently in bound states. The four fundamental forces are mediated via different bosons: The electromagnetic force (sun light, rainbow, batteries, red or blue poles of a lego toy train etc.) is mediated via the exchange of photons. The exchange of gluons (engl. glue = sticky material) yields the strong force which acts in large atomic nuclei and holds the protons and neutrons at very small distances together, although the protons carry same-sign positive electric charge and should therefore repell each other with immense force. The exchange of W⁺, W^- or Z⁰ bosons mediates the weak force, which is responsible for radioative decay or the burning process of the sun. Gravity is not yet understood on a quantum mechanics level, but presumably mediated vie the exchange of gravitons. Quarks participate in all three known fundamental interactions, while leptons only participate in the electromagnetic and the weak interaction. Quarks make up protons and neutrons, which in turn form atomic nuclei. The electron was discovered in 1897 and is by now the best known lepton. One or more electrons form the electron shell of atoms. Moving electrons represent an electrical current, for example in lightning in a thunderstorm. When rubbing balloons on a woolen jumper, we pull out electrons from its atoms, leaving an electrostatically charged balloon or jumper. Electrons and electrical currents are omnipresent these days and dominate our modern life. The partner of the electrically charged electron is the electrically neutral neutrino with only a tiny mass. Quarks and leptons come in different varieties, which appear to differ only in their respective mass. Otherwise they have identical properties. The varieties are called families or generations. The electron is a member of the lightest, first generation. The muon (μ) is the charged lepton of the second generation and has its own partner lepton, the muon neutrino. The third generation lepton is the tau with its corresponding tau neutrino. The muon is approximately 200-times heavier than the electron. Consequently, it decays in about 2.2 microseconds into an electron and two neutrinos and is therefore difficult to study. Very fast, highly-relativistic muons, however, can be studies easily, as their lifetime observed from a laboratory rest system appears to be stretched by the factor βɣ with β=v/c and ɣ=1/sqrt(1-β²) (time dilatation). Via this relation of the special theory of relativity, Albert Einstein managed to explain the muon paradoxon, i.e. the question why muons, which are created at an altitude of about 13 km, even at the speed of light should only travel about 660 m and should hence not reach the ground. Some muon properties are summarised in the following. Further information can be retrieved from a review article by the Particle Data Group. Muon mass m = 105.7 MeV, i.e. ~ 200-times the electron mass lifetime τ = 2.2 10⁻⁶ s 1936 discovered by Carl D. Andersen and Seth Neddermayer in studes of cosmic radiation using a cloud chamber. Muons can be produced for example in the decay of pions, which are produced in vast quantities in proton collisions with matter, for example the atmosphere. Muons