The development of relating negative space to positive space was examined in children between 18 and 36 months of age (N = 64). Children were presented with a series of compound objects, shapes embedded in shapes, and asked to match the inner aperture of the object to a corresponding tower. The results revealed that matching the negative space of an aperture to the positive space of another object is difficult even for the oldest children. This difficulty was influenced by the shape of the compound object, specifically the shape of the inner aperture in relation to the shape of the solid surround. The current study provides new information about the ability of children to solve fitting tasks.