Answer:
The simple answer is "bumping into each other".
In collision theory, as the particles move about near the surface of the liquid, occassionally, a pair will crash into each other with enough force, and at the proper angle as to create the escape momentum required to allow the molecule to eject through the surface tension and become airborn.
Read more: What are the particles doing when a liquid is evaporating? | Answerbag http://www.answerbag.com/q_view/93716#ixzz1HUzytoQf
Hope this helps (: