The pencil-like scribbles on your white mugs are often the result of a combination of metallic utensils and certain types of dishwasher detergents. When metal items, such as forks or knives, come into contact with the surface of the mug during the washing cycle, a chemical reaction can occur. This reaction is exacerbated by the alkaline nature of some dishwasher detergents, leading to the formation of grayish or blackish marks that resemble pencil scribbles.