Well, during the day, the water, as well as the surfaces surrounding the water, are heated by various thermodynamic processes: conduction, convection, radiation, etc. This in turn warms the water molecules in the lakes, streams, rivers, and oceans, thereby transferring heat (their kinetic energy) to the water molecules, which in turn receive that energy from the surrounding surfaces, or directly via radiation/insolation from the sun. When the water molecules attain enough energy, some of them attain enough energy to escape the surface of the liquid and enter the gas phase. Hence, as water is heated, more and more water molecules attain enough kinetic energy to enter the gas phase.