heat means temperature increases & a solid is heated ->  the temperature of the solid increases  the temperature of the solid increases & a solid is a kind of substance ->  the temperature of the solid substance increases  the temperature of the solid substance increases & as the temperature of a substance increases , the molecules in that substance will move faster ->  the molecules in the solid substance moves faster  the molecules in the solid substance moves faster & faster means an increase / higher in speed -> the molecules in the solid substance increases in speed