Ever since the muon was discovered physicists have wondered why it exists. Are there mathematics that require its existence for consistency? Are electrons and muons made of more fundamental stuff, with the muon being an excited version of the electron? We now know that each of the basic constituents of stable matter (the up and down quarks, the electron and its ghostly neutrino) is accompanied by two heavier, unstable versions. Flavor Physics aims at determining the properties of these particles with high precision, hoping to uncover some rationale for their very existence.