The present paper presents a tutorial on the solid effect (SE) as a mechanism for dynamic nuclear polarization (DNP). DNP is a method to orient nuclear spins in a solid. For this purpose, this solid is doped with a small amount of unpaired electron spins and a microwave field is used to transfer the orientation of the electron spins to the nuclear spins. Three different mechanism may be responsible for successful DNP: the Overhauser effect in metals and liquids, the SE and thermal mixing (TM) in insulators. The fundamental process of the SE are flip-flop transitions between the electron spins and the nuclear spins induced by the microwave field. In this paper the SE is approached in a similar way as in the description of cross polarization and coherence transfer between nuclear spins.