When cacao seeds, the raw material for chocolate, are crushed over a fire and kneaded well, they become a thick cacao paste. This is then mixed with cacao butter, sugar, and milk, and cooled in a mold while stirring to form chocolate.
Cacao paste contains a lot of fat, or oil like butter or margarine. This oil clumps together when the temperature is low and melts when the temperature is warm. Chocolate also contains a lot of oil, which causes it to melt when the temperature is high. This is the same reason why butter melts when it is heated.
Recently, however, a new type of chocolate has been developed that does not melt easily. This chocolate is made by breaking the chocolate into small pieces and wrapping each piece in sugar.