Strange makes a small noise of affirmation as Billie shittalks her dad. Same hat, Billie, dads are terrible. He's about to continue on the invasive question like when Billie asks hers. He gives her a bit of a Look: he knows she doesn't actually think that and is probably trying to get him to back the hell off. But that only makes him want to continue bothering Billie. This is slowly morphing into a battle of wills and Strange does not want to lose.

Besides, if she spends all her time asking him stupid questions like that, then he doesn't have to answer anything of substance!

"As far as I can tell, we don't. Though as I haven't actually seen any elves give birth, it's potentially possible." However, Strange's super dry tone plainly implies that no, it's not possible, that's a dumb question and you know it. "Was your other form always that size or did it grow with you as you grew up?"