In an almost quantum mechanics kind of a way, the idea of Ho'ill factors future possibilities into present Halachik realities.  This idea is applied here both in the laws of Challah teme'ah on yomtov, and the laws of eiruv tavshillin. The Chafeitz Chaim in Bi'ur Halacha 527 provides a clear and concise presentation of the technical aspects of these halachot. Tosfot describes the differences between future possibilities that while unlikely, are not abnormal, and those that are not normal and are not factored into present halachik realities. These ideas have profound implications for the ways we lead our lives.