i thought fire extinguishers very rarely exploded - unless they were defective with like a dent, heat doesn't seem to be reason in this case. did the broken fire extinguisher hit the professor in the head or did it hit the fire alarm which then hit the professor in the head? or maybe i'm just thinking about this to forget that best girl natsuki died terribly insignificantly and now tokiko is alone with no chances of happiness and giving up knowing that death just follows her?