The adorably named building blocks of protons and neutrons, quarks are never alone, existing only in groups. Apparently, the force that binds quarks together increases with distance, so the farther one tries to pry away a lone quark, the harder it will pull back. Therefore, free quarks never exist in nature. These fundamental particles come in six flavors: up, down, charm, strange, top and bottom. For instance, protons and neutrons are both made of three quarks, with protons containing two “up”-flavored quarks and one “down,” while neutrons have two downs and one up.
The up and down quarks have the lowest masses and are the most common flavors since the heavier quarks — such as the charm, strange, top and bottom — rapidly decay into up and down quarks. However, the heavier quarks could be produced in high-energy collisions, such as those in powerful atom smashers.