system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why do entire strands of Christmas lights not work if one bulb is damaged or taken out?
gpt
most lights are wired cheaply in serial, each light is connected to the previous light, remove one light and its the same as cutting the cable with a pair of scissors, the electricity cant flow. 

Electricity needs to flow in a complete circuit. If the electrical current can't get back to its source it doesnt flow at all, which is why it doesnt just light up all the lights prior to the broken one.

More expensive ones work in parallell (or have several parallelled groups of serial cables) these are evident by usually having multiple strands of cables. Removing one light will take out all of a group that are in that serial run but leave the others which are wired parallell to that run working.