Add Dimensions to your Brick
Dimensions represent controlled (independent) variables in your experiment. Usually, everything that is independently varied will get its own dimension. However, if you varied two things together (e.g., temperature and concentration of nitrate), you can make one dimension to represent what changed ("Condition" in this case), and add both temperature and concentration of nitrate as variables in that dimension. Each dimension needs at least one variable, which is usually a more specific description of the variable that you controlled along that dimension.