Flask

FIG. 1 is a front, top, and right side perspective view of a flask showing our new design; FIG. 2 is a front elevation view thereof; FIG. 3 is a rear elevation view thereof; FIG. 4 is a right side elevation view thereof; FIG. 5 is a left side elevation view thereof; FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof. The broken lines depict features of the flask that form no part of the claimed design.