"The rain itself...?" He just says it in wonder after Strange mentions it; sure, teleportation magic or magic that lets you go to the other planes exists in his world, but this makes it sound so much cooler, so much more interesting than any of that other magic. The fact that Strange is an adult who's actually fun to talk to only adds to his interest in the subject.

Which means that when he gets to the offer, Susan's eyes light up and he eagerly nods.

"D'you really think we can?! That would be so cool, I hope we can make it work! And if we find a way to make the magics work together then you can teach me even more spells! Or maybe I could even teach you some!" This is way better than stupid wizard school. He doesn't know how Strange does his magic exactly, but he's sure it's more interesting, just because it's Strange doing it.