The problem deals with a phase transition from solid to liquid. To the left we have ice and to the right we have liquid water.
For such change to take place, the process of melting must occur. During melting, a solid changes state from solid to liquid by taking energy from the environment.
In solid state, the molecules of the water are in a fixed state about their fixed lattice.
As the solid ice gains energy from the environment, it begins to melt.
Energy increase causes the molecules of the water to start flowing to form a liquid.
