system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do firefighters investigate a fire and figure out its source?

Like finding out if it was intentional or and accident and any other info useful to them.


EDIT: thanks to all who took the time to answer, I really understood even though I'm five.
gpt
What part of the house/structure was burned the most and how much /what pattern did the surrounding areas burn. They can extrapolate how long a fire was in an area based on how much damage it has done to certain substances (i.e. wood, metal, insulation). So, if there are four equally bad spots at four ends of the building, then their likely going to suspect arson.