Because they are fitted together in the stove so that they have not room to expand without breaking.
What effect has cold upon substances?
Cold causes them to contract or grow smaller.
Things expand unequally; do they also contract unequally?
They do contract unequally, and in this way may be broken.
Why are lamp chimneys often broken while the lamp is burning?
Because the heat of the flame causes them to expand unequally.
In what other way are they broken?
By a draught of cold air, or a drop of cold water, touching them, thus causing them to contract unequally.
Why are they sometimes broken when the lamp is not burning?