Scientific research shows that drinking water contains minerals and trace amounts of harmful heavy metal ions, such as cadmium, mercury, and arsenic. When water is heated, the solubility of certain calcium and magnesium ions decreases as the temperature rises, leading to an increase in the concentration of insoluble salts. When the water is concentrated to a certain extent, these substances precipitate out. These precipitates can enter the teacup when brewing tea. If tea is not finished or is left exposed to the air for an extended period, the tea polyphenols in the tea leaves oxidize with the metal elements in the tea residue, forming brown tea scale, commonly known as tea stains, which adhere to the inner wall of the cup. This tea scale contains various harmful metals such as cadmium, lead, mercury, and arsenic, making it a major culprit endangering human health. Long-term use of cups covered with tea scale can cause cancer. Tea scale contains toxic substances like cadmium, lead, mercury, and arsenic, as well as carcinogens such as nitrites. These substances adhere to the smooth surface of the teacup and enter the body's digestive system through drinking. They combine with proteins, fatty acids, vitamins, and other nutrients in food, forming insoluble precipitates that not only hinder the absorption and digestion of these nutrients but also damage organs such as the stomach and intestines. Additionally, cups that are not cleaned regularly accumulate more scale, which also contains large amounts of heavy metals, posing significant health risks. Furthermore, these oxides can cause lesions and functional disorders in the nervous, digestive, urinary, and hematopoietic systems, and may even lead to premature aging. In particular, arsenic and cadmium are carcinogenic, can cause fetal malformations, and pose serious health threats.