If you want to get the most out of a sketch, you need to leave big enough holes. Ambiguity creates the holes. It is what enables a sketch to be interpreted in different ways, even by the person who created it. One of the key purposes of sketching in the ideation phase of design is to provide a catalyst to stimulate new and different interpretations. Hence, sketching is fundamental to the cognitive process of design, and it is manifest through a kind of conversation between the designer(s) and their sketches.