Someone says, "there is a strong positive correlation between the number of firefighters at a fire and the amount of damage the fire does. so sending lots of firefighters just causes more damage." explain why this reasoning is wrong.

The reasoning that more firefighters cause more damage assumes to correlate that somehow the fire spreads with more firemen and women present. In fact the complete opposite is true. The more firefighters, the more people are there to rescue, handle the blaze, provide cleanup, etc.