An analysis of the heat quantity required to form a vapor bubble in a liquid is presented. High-speed motion pictures of boiling heat transfer in forced convection showing bubbles departing from a heated surface are analyzed and the net heat transferred to the bubbles is compared with the total heat transferred from the heated surface. It is found that the heat transferred to the moving liquid by the condensation of the bubbles is a negligible part of the total convective heat transfer. Therefore it is proposed that the high rate of heat transfer associated with surface boiling in a subcooled liquid is due primarily to the violent agitation of the quiescent layers of liquid adjacent to the heated surface resulting from the motion of vapor bubbles being generated there. From the literature two examples of similar processes are cited as evidence supporting the proposed agitation mechanism.