Some people think that since milk is a high-protein food and chocolate is an energy food, it must be beneficial to eat both at the same time. That was not the case. Adding chocolate to liquid milk will make calcium in milk react with oxalic acid in chocolate to produce "calcium oxalate". In this way, the originally nutritious calcium has become a substance harmful to human body, leading to calcium deficiency, diarrhea, delayed development of children, dry hair, easy fracture and increased incidence of urinary calculi.