People use two different techniques to make light bulbs flicker. One is primitive technology and the other is complex technology.
The original method is to install a dedicated flash bulb at any point on the light string. A common flash bulb looks like this: The extra piece of metal on top is a bimetal (see How Thermometers Work for more information on bimetals). Electricity passes through the metal piece and into the terminal post to light up the filament. When the filament heats up, it bends the metal piece, breaking the current and extinguishing the bulb. As the piece of metal cools, the bend returns to its original shape, the terminals are reconnected and the filament is relit, and the process repeats over and over again. Whenever a flash bulb goes out, the rest of the bulbs in the string are dead, so the entire string won't flash. Obviously, these bulbs don't have a shunt (if they had, the rest of the bulbs wouldn't blink), so when the flash bulb goes out, the rest of the bulbs go out as well, unless you replace the flash bulb. Today's more sophisticated light bulbs have 16-function controllers that allow you to control the light bulb in a variety of interesting patterns. In these systems, you'll typically find a control box that drives four separate mini strings of lights. The four light strings are connected staggeredly rather than one after the other. If you take the control box apart, you'll find that it's very simple in construction. It consists of an integrated circuit, four transistors, or triacs—one switch driving a string of lights. The integrated circuit only needs to turn on the triac to light up one of the light strings. By properly sequencing the triacs, a variety of lighting effects can be designed! To learn more about sequencers, see patent 4,215,277. For more information on Christmas lights and a variety of related topics, check out the links on the next page.