Yes, it is true that strawberries can make your teeth white because of their acid content.
To brighten teeth you can mash up a ripe strawberry and dip your toothbrush in it - but be sure to rinse thoroughly afterwards, using warm water to which a pinch of bicarbonate of soda has been added, to reduce acidity in the mouth.
Another way you can protect your teeth (though not whiten) is to chew a small cube of cheese after a meal. Casein - a milk protein - and whey protein in cheese have been shown to reduce the loss of minerals from tooth enamel.
Chewing cheese also stimulates saliva flow which neutralises the harmful acids that decay or erode teeth.
To look after your teeth it's also a good idea to chew sugar-free gum containing the sweetener xylitol.
Studies indicate that a xylitol-sweetened gum results in a reduced acidity response to carbohydrates. Research shows it also inhibits growth of decay-causing bacteria, and reduces the total amount of plaque on teeth.
To protect your teeth and keep them generally strong, drinking milky tea can also help. Milk is a great source of calcium while tea is one of the best natural sources of sources of fluoride which is known to prevent the loss of minerals from teeth.