All matter (the human body, air, water, etc) is made up of molecules which are made up of atoms which are made up of three types of particles: protons (positive charge), electrons (negative charge), and neutrons (no charge). Electrons orbit their molecules like planets revolve around a star. Occasionally an electron flies off and when it does, it leaves behind a positively-charged ion whose sole purpose in life becomes to fill the void left by its little lost electron. In other words: it wants to steal a replacement electron from another molecule. Due to differences in molecular structure (number of electrons in the outermost orbit, strength and structure of electron bonds, etc) some materials lose electrons much more easily than others