For example, fifth grade children sometimes tell us that the area of a 3 by 3 square is 9 square inches. Here children are simply applying rote formulas without understanding that the area of the square cannot be less than the area of the triangle. As children build their ideas about measurement of area, they often entertain a variety of ideas that capture some of the key ideas of measurement but violate others. Nevertheless it is important for them to try out their ideas so that they see why some won't work. Everything is length.
Children often believe that they can use rulers to measure area.
Both of these approaches are attempts to apply the single dimension of length to area which is two dimensional. Units can be different. Children often believe that it doesn't matter if the units are all identical. Cover need not be complete. Children often believe that although the units of measure should be identical, it doesn't matter if they don't completely cover a region.