Because that winter is so cold and they have no electricity or indoor heating to keep them warm, Mama would keep a small coal-burning stove on in the house all day (called a brazier). She would take it outside at night.
One night, Mark woke up and felt like some invisible force was choking him. He couldn't breathe. He starts kicking, and kicks Florah, who falls off the bed, hits the brazier, and starts screaming that she's on fire.
Mama runs out of the bedroom and throws urine on Florah. (This was a typical way to treat burns because they couldn't afford medicine.)
Mark explains that he's choking, and Mama rushes him outside. She yells at Papa to help her get the children outside.
It turns out, she had forgotten to remove the brazier that night, and they were all dying of poison gas fumes.