The particles that evaporate from a liquid first are the most energetic ones. The Maxwell-Boltzmann velocity distribution is an equation that describes this phenomenon, and it shows that in every substance, a fraction of the particles have a very large amount of kinetic energy.
An exponential function within the velocity distribution equation accounts for the particles that have enough energy to break free of the surface of a liquid. A solid object is one with the atoms or molecules arranged in a crystal lattice; a liquid has a looser arrangement; and, a gas has atoms with virtually no confinement forces. In a liquid, these energetic atoms are in a gaseous state, and evaporate from the surface.