Contact electrification has been a well-known phenomenon since B.C. 300. However, the origin of triboelectric charge and the charge transfer mechanism are not well understood. To date, the thermionic emission model, Schottky model, flexoelectric model, and intermolecular force model have been proposed for the contact electrification in conductors, semiconductors, and insulators. This article briefly introduces several important research results on the simple-seeming, but baffling, topic of contact electrification.