Now, all matter and mass is composed of very tiny units called atoms. Everything we see, we touch, we feel is composed of atoms. Now, atoms themselves are composed of nucleuses surrounded by electrons. Now, a nucleus is composed of two types of particles called protons and neutrons. Now, protons and neutrons have approximately the same weight. A neutron is a tiny bit heavier than protons, but for all purposes we can approximate that these guys have the same exact mass.