One thing that helps more than just looking surprised: ask the kid for further details. If it is something you know, you can catch anything they got wrong and ask “is it really like that? i thought it was different!” (keeping your tone in mind is essential, too)

If it’s something you don’t know, they’ll be more than glad to explain. Which can get tiring, but hey, it’s a small price for getting them interested in learning and sharing their knowledge