  One-dimensional model of a system where first-order phase transition occurs
is examined in the present paper. It is shown that basic properties of the
phenomenon, such as a well defined temperature of transition, are caused both
by existence of a border between the phases and the fact that only in the
vicinity of that border it is possible for molecules to change their phase. Not
only the model is introduced and theoretical analysis of its properties is made
but also the results of Monte Carlo simulations are presented together with the
results of numerical calculation of the distribution of energy levels of the
system.
