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How to whiten teeth with baking soda
Mix the baking soda with water or lemon (they both work fine).
In a small cup, mix half a teaspoon of baking soda with a few drops of either water or lemon. Be sure it is a paste, this way it will be easier to apply.
For applying the paste to your teeth you can use a toothbrush or just your fingers. If it is a bad stain you want to remove, it's recommended you apply the paste directly to the stain and let it set for about two minutes.