When it becomes difficult to distinguish between the figure and the background, such a figure is called reversible figure. Our attention keeps shifting from one object to another which is called as fluctuation of attention. Reversible figures are used in the experiments of fluctuation of attention. For example - in the Rubin's vase, we see two faces in the black colour and if we focus on the white colour, we can see the vase: Thus we can say that attention is not stable, it keeps shifting which can be understood by reversible figures.