Cavities and Edges in Breakfast Setting, Including Kitchen Sink Plumbing and Hatching Eggs - Left, Right and Centre

From Draw Like da Vinci, p. 41: Sharp lines are used to draw well-defined edges such as those of rocks, blades, boxes, machinery and the borders of cavities. Unlike a sphere, the edge of the cavity requires a definite, sharp line, especially in the foreground. A rim will require two parallel lines that vary in emphasis. Though hatching lines are used in a similar way to shape the internal surface of a cavity, the shading is darkest at the foreground edge. This gives the sense of the hollow.

Leaving light between the lines is the trick Leonardo used for shaping in shadows. There is as much shape in a shadow as on an illuminated surface. Filling in a cavity with unvaried shading flattens it, and transforms the hollow into an object. The cavity of a bowl or a box filled in will appear as a lid.