The textbook explains that -ing adjectives describe the qualities that things possess (what causes interest, excitement, or boredom), while -ed adjectives describe a person's mental or physical state (how one feels in response to things). For example, 'Chess is an interesting game' (the game causes interest) versus 'I am very interested in chess' (my mental state). Therefore, when describing things like serials, debates, stories, or looks, we use -ing adjectives. When describing how someone feels, we use -ed adjectives.