All matter is made up of atoms, which are themselves made up of charged particles. Atoms have a nucleus consisting of neutrons and protons. Typically matter is neutrally charged, meaning that the number of electrons and protons are the same. If an atom has more electrons than protons, it is negatively charged. Likewise, if it has more protons than electrons, it is positively charged.
Some atoms hold on to their electrons more tightly than others do.
How strongly matter holds on to its electrons determines its place in the Triboelectric Series . If a material is more apt to give up electrons when in contact with another material, it is more positive on the Triboelectric Series. The following table shows you the Triboelectric Series for many materials you find around the house. The relative position of two substances in the Triboelectric series tells you how they will act when brought into contact. Glass rubbed by silk causes a charge separation because they are several positions apart in the table.