Particle physicists at Europe’s CERN laboratory in Switzerland say they have observed bizarre new cousins of the protons and neutrons that make up the atomic nucleus. Protons and neutrons consist of other particles called quarks, bound by the strong nuclear force. By smashing particles at high energies, physicists have blasted into fleeting existence hundreds of other quark-containing particles. Until recently, all contained either two or three quarks. But since 2014, researchers working with CERN’s Large Hadron Collider have also spotted four- and five-quark particles. Such tetraquarks and pentaquarks could require physicists to rethink their understanding of quantum chromodynamics, or they could have less revolutionary implications. Researchers hope that computer simulations and more collider studies will reveal how the oddball newcomers are put together, but some wonder whether experiments will ever provide a definitive answer.To read the full story, see the 15 January issue of Science.