Answer: the water starts to evaporate.
Explanation: As the sun heats the water in the top of the puddle of water, the kinetic energy in the particles of waters starts to increase, and with this increase in kinetic energy, some particles can "escape" the corpse of water.
This is called evaporation, so as the sun heats up the surface of a puddle of water, the water starts to evaporate. (only on the top, because is the region that is being heated up and also is the region where the pressure is the smallest)