What is evaporation? 

The spontaneous change in which a liquid change into vapours (or gaseous) at the surface of the liquid is called evaporation. 
Evaporation may be explained in terms of the motion of molecules. At any given temperature, all the molecules do not possess the same kinetic energy (average kinetic energy is. however, same). Some molecules move slowly, some at intermediate rates and some move very fast. Rapidly moving molecules near the surface of the liquid, possessing extremely high energies are sufficient to overcome the intermolecular forces of attraction. These molecules tend to escape into space above the liquid surface and form vapours.