When you melt glass, the glass changes from a solid to a liquid. when the heat source is removed and the melted glass is allowed to cool, it changes back into a solid. the transfer of energy caused a change to the of glass.

The perfect word to fill the blank is "structure".  The transfer of energy caused a change to the structure of glass, especially in the molecular level. Transfer of energy has an effect on the molecules in motion. When energy is added, the molecules kinetic energy will increase and they will move faster and the structure will be less structured or packed.