You can sew a button when it comes loose, but fixing a broken zipper is a bit more complex. Like every element of your clothing, zippers wear down over time. If your fly gets stuck or the front of your jacket comes open when it's not supposed to, you may assume the garment is ready for the recycle bin. But with a few items you likely have at home, a broken zipper can be fixed.
Through constant use, both the slider and the zipper teeth can warp and become misaligned. If your zipper has trouble staying in place, break out your tool box. Get a derailed slider back on track by threading the bottom end of the teeth through the corresponding opening in the slider, using a flathead screwdriver for leverage if you need it. Once it's reattached, pull the slider up and down to join the teeth together.
To make sure this doesn't happen again, grab your pliers. Gently squeezing the openings of the slider—just enough to tighten them without making them too narrow to fit the zipper teeth—should solve your problem. This will also prevent the two halves of the zipper from separating after you fasten them.
If the slider is slipping down on its own, the issue may be with the zipper teeth. When that's the case, look for teeth that have bent out of place and use the pliers to guide them back in line. Teeth that appear to be in place but are incapable of holding up the slider may have worn away. You can give them back some bulk by painting them with clear nail polish. After applying the first coat, let the polish dry and test the zipper to see if it stays up. Keep adding coats until the teeth are substantial enough to hold the zipper together again.
A stuck zipper requires a different set of supplies. First, trying rubbing a pencil across the teeth. The graphite should act as a lubricant and make the zipper easier to open. If that doesn't work, try using other lubricating substances—like bar soap, lip balm, or Vaseline—to dislodge the snag. If there's something caught in the zipper, like a loose thread, you may need to remove it with a pair of tweezers first before you try opening it.
See these fixes in action in this video from Lifehacker.
Now that you know how to fix a broken one, the only zipper-related mystery left is what the initials YKK stand for. You can find the answer to that here.