Yes, It's Possible To Rescue Those Popcorn Kernels That Failed To Pop
No matter how thorough you are when making popcorn, a few lonely kernels will inevitably get left behind. But don't worry, you don't have to toss them out. If you have a little patience, you can save those kernels and prep them to pop at your next at-home movie night.
The trick is to get them a little moist. Popcorn kernels are finicky about when they'll pop, requiring an internal moisture level of about 14% before they'll do so. So your leftover kernels were likely too dry to properly explode in the microwave or on the stove. To rescue them, shake them in a jar or airtight container with a tablespoon of water, then let them sit so all the liquid can absorb. In a few days, you'll have kernels that are ready to pop.
You can check if your kernels are too dry by digging your fingernail into them — if you can't make a dent, but they still aren't popping, this method will likely do the trick.
Add or remove moisture from your kernels
While this can help, there's another potential reason your kernels aren't popping — they're too wet. If you dig your fingernail into a piece and end up denting it, you'll want to dry it out a bit before re-trying. All you need to do is spread your wet kernels on a sheet pan and let the moisture evaporate, which should take less than a day. While it's tempting to want to speed the process along by drying them in the oven, avoid doing this as it may impact the kernels' ability to pop fully. This is why it's essential to add no more than a tablespoon of water to dry kernels — any more, and you may just end up with soaking-wet pieces.
If you've tried both of these methods and still have no luck, the hulls may be too damaged to create the steam pressure needed for a pop. But popcorn kernels are pretty hardy, so most of those that are unpopped simply need this moisture adjustment.