The term "crumb mixture" refers to a combination of ingredients used in baking to create a crispy, crumbly texture on top of baked goods such as pies, cakes and muffins. This mixture typically consists of flour, sugar, and sometimes leavening agents like baking powder or baking soda. The proportions of these ingredients can vary depending on the desired consistency and flavor profile of the final product. In some recipes, additional ingredients such as butter or oil may be included to help bind the crumb mixture together. Once the wet ingredients are added to the dry in a baking recipe, the crumb mixture is used to top off the finished batter and create that signature golden-brown, crispy texture on the surface of the baked good.