Complex sentence: Ordinary matter, such as atoms, contains electrons and a nucleus consisting of two kinds of particles called nucleons: protons, which contain two up quarks and one down quark, and neutrons, which contain one up quark and two down quarks.
Split into simpler sentences: Ordinary matter, such as atoms, contains electrons and a nucleus. The proton contains two up quarks and a down quark, while the neutron contains one up quark and two down quarks.