This is the triumphant grin of a victorious jackanape. He catches the start of a sentence as she turns away, and is about to tap her on the shoulder when she realises it herself, and turns back to him.

He won't say it because it would take too long to write, but he is grateful beyond imagining that she is taking to this so well.

i should have figured out how to make a sign-to-speech translator, haha. we'll totally explore the whole planet! it's much brighter than mine.

He waits for a break in her alchemizing to hold the note out to her.