While there is never a good time for an off-track zipper, a broken lunch box zipper can be a special sort of annoyance. When it comes to keeping your food fresh, there's not much room for error. Frustrating as it may be, always keep a cool head when dealing with an off-track zipper. Once you learn to reset a broken zipper, you can apply the skill to all sorts zippers, not just those on lunchboxes.
Identify the zipper stop, the bit of the zipper that stops the pull tab at the end of the track. It lies near the bottom, or beginning, of the zipper track. Stops come in the form of small rectangles or sets of two-to-four prongs at the end of the zipper track.
Pry open the tabs or prongs that secure the stop with a pair of needle nose pliers. Remove the stop from the zipper track.
Remove the head of the zipper — the piece that secures the pull tab to the zipper track — with the needle nose pliers. You'll have to wiggle a bit to pull the head off one side of the track. The pull tab is likely secured to the head; it's okay to remove them both.
Align the head of the zipper so that the teeth on either side of the zipper track line up with the two holes on either side of the head. Using the pliers, hold the head at the bottom of the zipper track -- below the spot where you removed the stop -- between the two sides of the track.
Pinch the zipper's track tape, the fabric surrounding the teeth, with a pair of tweezers. Starting on one side of the track, carefully work the tape inside the hole on the corresponding side of the zipper head. When you've got the tape in one hole, move on to the other side of the track.
Slowly slide the zipper up the track. If the action works, you have successfully gotten the zipper back on track. If it doesn't, start back at the beginning of the track and reinsert the tape again. It may take a few tries to get it just right.
Reinstall the zipper stop by re-clamping the prongs or tabs in their original position.