Our toothbrushes are made highly water-resistant using two methods. First, we use our patented heat-pressing method to increase the carbonization and caramelization of the bamboo surface, creating a smooth finished surface. As a result, you may notice that there is some residual yellowing of the water surrounding the toothbrush if it is left to soak. This is due to the natural color of the toothbrush and is not something to be alarmed about. A food-grade oil coating on the surface of the toothbrush provides an additional layer of water resistance.