The first law of thermodynamics basically states that no system can expend more energy than it has already plus any energy input into it. Or, in simple English, there’s no such thing as a free lunch. Dragons, which typically expend vastly more energy than they take in, clearly violate this law and are, therefore, scientifically impossible. Try explaining this to Cecil, the official sidekick of Archmage Serena of the sisterhood of the Future Mages, and see what happens. Better idea: don’t! Annoying a giant flying reptile with huge razor sharp claws and a built-in flamethrower by questioning its current existential state is probably not a good idea, especially from the Darwinian perspective.