The Nightfury looks very serious as he mulls over this explanation. His eyes are wide as the cogs turn and turn in his mind — which isn't exactly built for this sort of conversation. But he tries.

What's an Earth? And a Heaven? he wonders next.

There are, surely, an endless stream of questions that the dragon could come up with, but he does want to understand just what this viking — human — is. He doesn't smell right. There's something off about his scent.