The ornamental design for a fountain, as shown and described.
FIG. 1 is a perspective view of a fountain showing my new design;
FIG. 2 is a side view of the fountain of FIG. 1;
FIG. 3 is a top view of the fountain of FIG. 1; and,
FIG. 4 is a bottom view of the fountain of FIG. 1.
The broken line showing of a portion of the fountain in FIGS. 1 and 3 are included for the purpose of illustrating and forms no part of the claimed design.