Leading theories of lying disagree on many points, but they agree on the following assumption: lying essentially involves asserting. The possibility of lying by omission poses a challenge to that shared assumption. To lie by omission is to lie by not asserting. This paper is the first experimental investigation of whether lying by omission is conceptually possible, according to our ordinary, shared lying concept. Overall, our results support, without proving, that it is not possible. Based on the present findings, we hypothesize that to the extent that people are tempted to call an omission a "lie," it is for lack of a better word. When provided more flexibility to express themselves, almost no one in our studies counted an omission as a lie.