It's a Loctite Super Glue Liquid Pen ($1.99 at Walmart). I followed the instructions on the package, applied the glue to the key's back, then used tweezers from a Swiss Army knife to set the key in it's place on the keyboard, and applied pressure for the time described in Loctite's instructions. The key's arn't as firm as those that didn't fall off, but they're fairly close.