Subject Variables as IVs
There are variables whose effect on a DV might be of interest to you but that you cannot manipulate. For example, you might be interested in the role that gender plays in the response to TV violence. Thus, you might have a group of children who view a violent cartoon and a group who view, as a control, a nonviolent cartoon (IV - type of cartoon). Then you measure how aggressively the kids act towards a stuffed animal after watching the show (DV). You test boys and girls (IV). The design would have two IVs: Cartoon and Gender.

However, you did not manipulate Gender. You had to take the children with the gender they came with. This is a subject variable. You can't really say that XX versus XY chromosomes gave you your result. What about culture? Subject variables are not as clear-cut in demonstrating causality as many other IVs.