A couch is the standard unit of fire progression.
At the Museum of Science and Industry, a small display about combustion
shows how quickly a a couch will burn under different conditions.
First, a warm bead in the corner. Flames suddenly slip from the cushions
Step one: visibility. The first panic.
You try every irrational thing to put it out: moving the couch,
taking off the cushions, stepping on them.
A certain point-- unnamed in the video, but painfully obvious--
is the point of no return. Step two. The couch
has ceased to be a couch, just soft debris,
the inside of a flame's mouth. When a couch reaches stage two,
the whole house does at once. So then you wait for stage three.