Please don't think I'm crazy....Oh wait, I am.
So it is thought that birds may have evolved from reptiles. The main differences is that they have a four chambered heart, are endotherms, and have feathers and wings. They have some other adaptations like beaks and hollowed bones etc.
The main reason, that I have heard, that dragons would not exist is that they are too heavy to fly.
Then I came across another idea. What if dragons (giant flying reptile-birds) could somehow heat up the air inside of them, makeing them lighter. I mean they can exhale fire, right. Also, their size may have been exaggerated a little.
Anyways, is there any organism in the world that can heat up air inside of them for the purpose of becoming lighter? If not, is there anyway that this could be done?
One more thing. How small would a dragon have to be to fly without the internal heated air?
Have at it you Biology experts.