The Ancient Greek philosopher Leucippus of Miletus and his disciple Democritus of Abdera developed in the 5th century B.C. the idea that the world consists of a number of tiny, prime particles, the atoms (in Greek: 'indivisible'). As of today, we know 118 atoms, classified into the periodic table of the chemical elements. We know actually that these atoms are made up of yet smaller particles: the protons and neutrons, which form the nucleus, and the electrons surrounding it. Protons and neutrons are in turn made up of 'up quarks' and 'down quarks'. According to our present knowledge, our matter consists therefore of three prime particles: the electron, the up quark and the down quark. To visualise an atom requires some imagination. Since the atom is about 100 000 times bigger than its nucleus. If the atomic nucleus were the size of an apple, then the atom itself would have a diameter of around 10 km, following the comparison by physicist Harald Fritzsch. Even though we perceive matter as solid, the individual atoms are to a large extent empty: "Even a diamond consists mostly of empty space", says Fritzsch.