Keith bit his lip, a habit from when he was a toddler, and rocked back and forth on his heels, trying to think up a good lie. Finally he decided to tell the truth.
Well, part of it anyway.
"Well sir, that's uh- for a friend," He said. Not entirely untrue. Christy was a friend...Kind of. No, not really. He decided he didn't really like her after all. Keith finished this thought by telling himself if he got out of this, Christy was paying double for her alcohol and potions. "She's been feeling...Well not good lately. She told me that this stuff will make her better, if she takes it often enough." Good. Play the ignorant little kid angle. Though this professor looked a shade to street smart to not see past that particular little ruse, it never hurt to try.
All throughout his little conversation, a tingle was going up Keith's spine. If he had been a more astute person, he would have realized it was coming from the small gem studded ball laying on the floor. Also, he would've noticed a faint, near invisible clear mist smelling faintly of paint. He would then have deduced that the mist was most likely some kind of dark magic, and it was targeting his nervous system, hence the tingles. He then would've picked it up and chucked it out the window, into the lake, before it could spread and do any real harm.
Yeah, it was too bad Keith wasn't a very astute person.