The ornamental design for the hand warmer, as shown and described.
FIG. 1 is a front perspective view of the hand warmer showing our new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a rear perspective view thereof.
The broken line shown in the drawings are included for the purpose of illustrating portions and environmental elements of the hand warmer and forms no part of the claimed design. The strap element, which is disclaimed and forms no part of the claimed design, is not shown in FIG. 6 for ease of illustration and so as to not obstruct any part of the claimed design.