The answer is nothing less than shocking!
People often go to the extremes to take care of their skin, their teeth, their health, etc. They seem to be attracted to all kinds of chemicals and artificial products, yet never bother to learn the old Italian granny way of staying natural, staying healthy. Coming to the teeth specifically, everybody knows the basic teeth whitening tips like the usage of lemon, baking soda or even banana peels, but I bet you didn't believe me when I claimed that you could achieve the same using only a slice of bread.
Well, here you go.
Mind you, this is just a teeth whitening technique. You still need to brush your teeth regularly.