Generally, there is a layer of paint on the porcelain bowl, which will fall off after a long time, and then the bowl will turn black. The quality of porcelain bowls is poor, the surface smoothness is not enough, and it is easy to accumulate tiny particles such as pigments, which is not easy to clean. Or the food you put will be colored after soaking, and it is normal to leave color in the bowl after boiling. You can wet the bowl and wash it with proper amount of toothpaste.