The sponge tool changes the saturation of the colors in an image in the area it is applied to. The sponge tool can be made to saturate or desaturate the image based on the settings in the tool options bar. If you set the tool to saturate you will notice that the colors or hues will become more intense where the sponge tool is used to stroke the image. If the tool is set to desaturate brushing the image will achieve the opposite effect resulting in reduced intensity in the colors. If you keep in mind that a fully desaturated image is grayscale or black and white, then desaturation with the sponge tool is simply moving the colors closer to shades of gray. Like the blur tool the sponge tool can be used to create emphasis on a particular section or subject in an image by making the subject more saturated with respect to its sorrounding.