Use salt and toothpaste to wipe the rust off the cup. First, soak the scrubbing tools such as gauze or paper towel, then dip the soaked gauze into a small amount of edible salt, and wipe the tea rust inside the cup with the gauze dipped in salt, which has a significant effect. Squeeze the toothpaste on the gauze and scrub the stained cup with toothpaste. If the effect is not significant, squeeze more toothpaste to wipe it. After washing the cup with salt and toothpaste, it can be used.