Ever had to throw away an entire string of Christmas lights just because one or two bulbs weren’t working? You’re not alone. Some Christmas lights just won’t work with a single burnt-out bulb, while others will just keep on glowing all winter long! What makes some lights different from other lights? Turns out, it’s all in the manufacturing — and how the lights are wired at the factory. Here’s how those massive strings of glowing festival decor are made.