Because the heated air bursts the cells with such force as to throw off small pieces of the burning wood.
What kinds of wood snap the most?
The coarse-grained kinds, like chestnut; while the finer-grained kinds, like walnut, seldom snap much.
Why does dry wood snap more than green wood?
Because the sap in the pores of the green wood dries up as the wood becomes dry, and its place is filled with air, so that there is more air in dry wood than in green wood.
Why does smoke rise in a chimney?
Because the air in the chimney is heated by the fire, and as the air rises, it carries the smoke with it.
Why does a chimney smoke when the fire is first kindled?
Because the cold walls of the chimney cool the heated air so rapidly as to prevent its rising to the top, and the smoke then sinks back into the room.
How are houses heated with hot air?