Fairy Light Bulbs. How Do They Work?

We have a set of "Snowflake" fairy light bulbs that are old enough to be almost a family heirloom. So old, in fact, that they use incandescent bulbs instead of LEDs. In the last video we found some replacement bulbs, OK, but I've always wanted to know how it is that when one bulb blows all the other's stay lit. Lets see if we can find out.