Matter is made up of atoms, which are the fundamental building blocks of all physical substances. Atoms consist of a nucleus made up of positively charged protons and neutral neutrons, surrounded by negatively charged electrons. These atoms interact and combine to form molecules, which make up the various kinds of matter we see around us. At a more fundamental level, protons and neutrons themselves are composed of smaller particles called quarks, held together by gluons. Thus, matter fundamentally consists of elementary particles known as quarks and leptons (including electrons) that form atoms and molecules, taking up space and having mass.