Sketch means that you should clearly describe the relationship between black, white and gray of the pattern you represent, and express it with points, lines and surfaces. Use brain processing to analyze where the dark side and bright side of a lunch box are. The basic color of disposable lunch box is gray, the whitest place is light gray, and then the dark parts are deepened in turn, but the darkest place should not be dark because its inherent color is white. The things around the disposable lunch box are basically the brightest in a painting if compared. You can draw a picture directly.