Friction-driven static electrification is familiar and fundamental in daily life, industry, and technology, but its basics have long been unknown and have continually perplexed scientists from ancient Greece to the modern high-tech era. Despite its simple manifestation, triboelectric charging is believed to be very complex because of the unresolvable interfacial interaction between two rubbing materials. Here, we reveal a simple physics of triboelectric charging and triboelectric series based on friction-originated thermoelectric charging effects at the interface, characterized by the material density (rho), specific heat (c), thermal conductivity (k), and Seebeck coefficient (S) of each material. We demonstrate that energy dissipational heat at the interface induces temperature variations in the materials and thus develops electrostatic potentials that will initiate thermoelectric charging across the interface. We find that the trends and quantities of triboelectric charging for various polymers, metals, semiconductors, and even lightning clouds are simply governed by the triboelectric factor xi = S/root rho ck. The triboelectric figure of merit is expressed with the triboelectric power K = xi root t/pi, of which the difference can be maximized up to 1.2 V/W . cm(-2) at the friction time t = 1 s. Our findings will bring significant opportunities for microscopic understanding and management of triboelectricity or static electrification.