Fire dragons are the most commonly known type of dragons in the world.  They usually have brown or red scales.  Here is how they breathe fire.  They have an extra organ in their belly that contains white phosphorus which is a chemical that will light on fire when exposed to air.  This may be surprising but dragon fire comes out of its tongue, not its mouth.  The tongue is a tube that connects to the organ of white phosphorus.  The tongue closes up when the dragon does not mean to breathe fire.