1. baking soda: every time you brush your teeth, add a little baking soda to the toothpaste. After brushing several times, the teeth will turn white slowly. This method is mainly aimed at the yellowing of teeth caused by rust.

2. Vinegar: Both white vinegar and aged vinegar are acceptable. Put vinegar in your mouth 1 to 3 minutes, then spit it out and brush your teeth.

One disadvantage of this method is that it will make your teeth numb and sour, so it is not recommended to use it often unless it is an emergency.

4. Lemon and apple vinegar: Lemon can remove tartar, and apple vinegar can whiten teeth, so the combination of the two can whiten teeth and improve tooth yellowing.

5. Laser whitening: a medical method of tooth whitening, which removes tartar and substances attached to the surface of teeth to achieve the effect of tooth whitening.

6. However, this method cannot be used frequently, and it can be done at most 1 or twice a year.