One of the active ingredients in many over the counter teeth whitening kits is something called malic acid. It is a naturally occurring chemical that has astringent qualities and also acts as a gentle bleaching agent.
Strawberries contain relatively high levels of malic acid so munching through just a few of these tasty little treats will have a positive effect on stained teeth. Some people even advocate using strawberries as quick-hit home remedy on your teeth. Crush up a strawberry, mix it with a little baking soda and gently rub the mixture onto your teeth. Rinse off after a few minutes and your teeth will be visibly whiter.