Why do the christmas tree lights still burn when some lights are burned out ?

There are two ways to wire lights.
In Series. ..........Thats when one burns out and they all won't light. (cheaper?)
or Parallel. ......Thats when one can be bad and the rest keep lit.
Here are some very simple explanations and pictures of the circuits.
http://www.berkeleypoint.com/learning/parallel_circuit.html
Choose the right one if your going to use LEDs.
GL Jim