I understand why an impure solid will have a decreased melting point, however, in my recent organic chemistry lab section I mixed two finely ground pure solids together and observed that the mixture melted at a lower temperature than either pure solid. I understand this in principle, but whenever mixed melting point theory is taught, it is taught from the perspective that impurities reduce the strength of a crystal structure. However, in my experiment I had two *solid* pure substances, mixed together. The crystal structure could not possibly be disrupted, as the compounds had never mixed in liquid form. So my question is: how does that work? Is it just the case that a small amount of one or both of the pure substances is melting at the edges of the crystals, and that when this happens, this tiny amount of impure liquid can dissolve more of the two pure solids, and that this is what causes the melting point depression?