Miranda asks Professor Mousebane if she may have one dose of Veritaserum to use on herself for an experiment?

What's the experiment?

"Testing my Occlumency. It probably isn't that good but I'd prefer finding that out with Veritaserum and a friend asking me things than with a mind affecting charm."

"Veritaserum is very expensive, even a drop of it."

"...how expensive?"

"I think around a Galleon, not that you can buy it by the drop..."

"Would you sell me a drop, if I came up with a Galleon for it?"

"...sure, if I can supervise that that's what you're using it for."

"I was really hoping to be alone with nobody but Karen for the experiment. Mind stuff makes me nervous, that's why I studied Occlumency to begin with."

"That's understandable, but you can also understand how we can't have students using it on each other."

"You could give me the dose and I could go in a room with Karen and have my dictation quill set up and when I come out you can read the transcript and she'll vouch that I took it?"

"Sure, that sounds safe enough."

"Thank you. I'll see if I can afford it."

"You do that, dear."

And Miranda runs along and assesses her finances and cannot currently afford it unless she spends Aaron's Christmas present. (It's really conspicuous that the Ways have more of a budget than she does given Aaron's habits. However, it also means she has a Galleon.) She dithers for a bit about whether she wants to spend it, waits a week rereading all the Occlumency books in case there is more progress she can make without what information failing the test might give her, and goes and offers Professor Mousebane the money after setting up a time with Karen.

And Professor Mousebane bites her lip and asks yet again exactly what Miranda is planning to do with it - and, did Miranda know, first-years really shouldn't be able to learn Occlumency - and gives her the drop.

Miranda reiterates what she is planning to do with it and leads Professor Mousebane to an empty classroom and demonstrates the dictation quill and disappears into the room with Karen.

Karen comes out with the transcript ten minutes later giggling her head off, probably at the increasingly ridiculous answers Miranda has been supplying to her questions.

And Professor Mousebane is satisfied that Miranda probably did not sneak the dose away to spike a classmates' drink.

And Miranda's walking on air all the way back to her common room.

"Hey! Inherit ten thousand Galleons or something?"

"No, I'm actually down one."

"Oh?"

"Veritaserum's expensive."

"What're you doing with Veritaserum?"