Harry is also there, looking a bit nervous.

"Hello, hello. Let us get started. I trust you both have had a look at the mental exercises?"

"I looked at a couple." He also had homework and needed a human amount of sleep.

Cam nods rather than specify in an unavoidably comparative fashion how much more he has accomplished than this twelve year old.

"Then the next step is to learn what a legilimency probe feels like. The best legilimens can be very subtle, but less sophisticated probes can be noticed if you know what to look for. This is also," he nods at Cam, "the aspect I am most confident is possible without our kind of magic. Now, does either of you wish to go first? I will of course exit your mind as soon as I have entered it and do my best not to look at anything too closely."

"Uh, what is your credence in the hypothesis that I have actually learned to do this overnight?"

"Oh, it's perfectly alright if you haven't; many people find they have more success with the exercises once they have a sense of what they're supposed to be countering."

"Sure," says Cam, "but I'd in fact rather you not read my mind, so if I'm likelier to manage this after another week..."

"You are of course welcome to wait another week, though be aware that many people reach a point of diminishing returns with the exercises until they've had some practice with a legilimens."

"Isn't there a less invasive option? The book mentioned a truth potion, which in principle with a cooperative interlocutor wouldn't be an issue."

"Ah, yes, veritaserum. Many people find the sensation of being compelled to speak the truth more unpleasant than that produced by legilimency, but if you wish to try that instead some can be procured. How about you, Harry? Do you have a preference?"

Shrug.

"I don't usually go around lying anyway and if you ask me invasive personal questions I can probably fill my mouth with caramel and then also tell everyone you did that."

Dumbledore seems amused at the mental image. "Very well, then. I believe our potions master has some in stock; I'll ask him to send it up." He writes a note on a bit of parchment, starts a heatless green fire in the fireplace, says "Professor Snape's office" and tosses the parchment in. 

"And now we wait. Harry, would you like to try the legilimency method in the meantime? I expect you in particular will benefit from being able to recognize legilimency."

"It's not going to hurt or anything, is it?"

"Certainly not. It will merely feel like your mind being drawn to the memory of, hmm, how about your breakfast this morning?"

Harry nods. "Okay."

"Excellent. Take a moment to clear your mind."

"Alright. I will make the attempt in three . . . two . . . one . . . now."

Harry blinks and shakes his head. "That did feel odd. It was like being reminded of eating breakfast but not being reminded by anything."

"A very astute description. On the next attempt, you may wish to try . . ." he goes into some rather metaphor-heavy descriptions of mental motions Harry could try making. Cam, having already read about this stuff, may or may not find it interesting.

Cam does take notes of where the metaphors differ!

They clearly come from the same general base of ideas, but there are some differences too. They try a couple more times; Harry is doing well for a twelve-year-old but that's a big caveat and it's going to take him a while to get the hang of it.

Eventually the fireplace briefly fills with green fire again and acquires a small crystal phial with a few drops of clear liquid in it. "Ah, here we are," Dumbledore says, retrieving it with a flick of his wand.

"It just occurred to me that this substance might be expensive and I can make you, like, whatever's hot on the commodities market right now for it."

"That would be appreciated. I'm told you can't make magical substances, but a great many potions require esoteric nonmagical ingredients. Could I have a bag of scarab beetle eyes, already removed from the beetles?"

Big bag o' eyes.

"Excellent, thank you." Bag of eyes goes on a shelf next to the snowglobe containing a tiny thunderstorm, and Cam can have the phial. It has a dropper cap.

"One drop generally makes it impossible to lie, three makes it impossible to withhold information and is correspondingly more difficult to throw off."