The ornamental design for a sponge, as shown and described.
FIG. 1 is a right-side view of a sponge showing a first embodiment of my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top-right-front perspective view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right-side view of a sponge showing a second embodiment of my new design;
FIG. 6 is a front view thereof;
FIG. 7 is a top-right-front perspective view thereof; and,
FIG. 8 is a bottom view thereof.
In each of the first and second embodiments, the left-side view is a mirror image of the depicted right-side view and the rear view is a mirror image of the depicted front view.