The most annoying thing about Christmas lights is when one bulb blows, the whole string stops working. There's no easy way to tell which one is out when it's a huge strand, so it can take forever to fix.
Mehdi Sadaghdar figured out that zapping the Christmas lights with a piezoelectric igniter from inside a multipurpose lighter fixes the strand. The quartz crystal inside the igniter generates piezoelectricity, and the voltage is high enough that it shorts the busted bulb, even with a broken filament, and creates a path for the electricity to continue flowing down the series circuit.
However, by doing this, you are increasing the voltage across the remaining lights, so if you have more than one or two blown bulbs and try this trick out, you might just have a full string of blown bulbs later. Check out Mehdi's video to learn more about how it works.