Quarks are elementary particles that are one of the two types of fermions, the other being leptons. Quarks are the only elementary particles that interact in all four fundamental interactions of physics. There are six flavors of quarks: up, down, strange, charm, bottom, and top quarks. The antiparticles of quarks are called antiquarks. Quarks have spin 1/2, baryon number 1/3, and electric charge 1/3 or −2/3. Properties and quantum numbers usually can be found by direct experimenting but confinement of quarks stops any chance of precise measurement.