What happens to the other bulbs if one bulb burns out in a simple series circuit of holiday lights?

In a series circuit there is only one path for current to flow. If one bulb burns out, it breaks the path (creates an open circuit), so current can no longer flow and all the bulbs go out. This is the classic problem with old-style series holiday lights. In a parallel circuit, by contrast, each bulb has its own path, so one burning out does not affect the others — which is why modern wiring and most household circuits use parallel connections. The behavior when one component fails is a key way to tell series from parallel.