Have you ever decided to bake and got the whole process started and realized you are missing brown sugar? It happens to me all the time. Luckily there is a solution:
1 cup granulated sugar + 1 tablespoon unsulfured molasses
Just place the two ingredients in a bowl and mix really well with a fork. It will be clumpy at first but if you continue to mix and fluff it will eventually resemble store-bought brown sugar.
Cool note: In the process of refining sugarcane there are two products produced, white granulated sugar and molasses. Commercially produced brown sugar simply puts the two back together; less molasses for light brown sugar and more for dark brown sugar.