Ohaguro is the custom of dying teeth to… black.

Although it might seem that beautiful teeth are white teeth, it occurs that not to everyone.
There are cultures where bones, as an internal part of the body, are considered unclean and should not be shown. Displaying in their natural color is disguisting. This belief comes from Buddhism and is fortunately a relic of the past. However, there are still places in Asia where we can meet young and old people with black teeth.