The melting point of a solid is the temperature at which it changes state from solid to liquid. The freezing point (or crystallization point) is the temperature at which a liquid changes to a solid. For certain substances the melting and freezing point are not the same and the freezing point is lower than the melt point. This behaviour is called hysteresis. At the STOP point (freezing point) the dye can't move. At the melt point the dye moves and indicates a threshold breach.