And if she goes over and peels the tape back and looks in there?

The box is oddly deep! Twice as deep, in fact.

She drops a pencil in it. If nothing unexpected happens, she picks up the box and dumps the pencil out.

Nothing unexpected happens when she drops the pencil in.

As soon as she tilts the box, though, its visible interior twists dizzyingly and the spell breaks; it's six inches by six by six again.

Well, that was rather nauseating. And also annoying. And hopefully due to a weakness in her incantation rather than a fundamental limit of space-warped volumes. She draws a line down the inside of the box from the top of one side face to the bottom, and goes back to French vocabulary. Dungeons and Dragons night rolls around before she has it done.

The campaign ends with a tremendous party-splitting showdown; Xavier squirrels away Brenda, Cole, and Joseph in one room and Sanjay, Alec, and Margaret stay behind, with Xavier darting between rooms to adjudicate things and move enemies and try to reply before the other half has finished arguing about their next moves. He's pretty good at it. Finally they converge and defeat the bad guys.

It's extremely theatrical and terribly exciting and the long time they get to argue about their moves lets them be more tactically sophisticated than they can usually get. Margaret is immensely pleased and says so.

"Glad you liked it!" says Xavier. "Can I count on you for the next one? Starts next month, I need a bit to prep it."

"Yeah, totally! Same system, different premise?"

"Yup! And you can have right of first refusal on being the wizard this time if you want, Cole's going for a weird cleric prestige class this time - gonna start at level six."

"Sure, I'll be a wizard! Hey Cole, tell me more about this weird prestige class!"

Cole opens up the nearest copy of Complete Divine and shows her!

"Nifty! How about you, Brenda, got a concept yet or still thinking?"

"I'm gonna be a fighter," she says. "Level six is enough to come out of the gate with a few feats, I'm not sure about exact build though."

"Yeah, I'm going to need to figure out specializations too." She packs up her bag and gets ready to set off.

"See you around!"

There's only one evening between game night and trivia night, but that's long enough to do a bit of magic research. She has a taped-shut box with a marker line on one inside face and a new incantation, translated from "Make the inside of this box twice as large in the dimension I marked along with ink, so that the inside of the box still approximates a rectangular prism but no longer approximates a cube and the line is twice as long, leaving the other dimensions and the outside unaffected."

(That incantation needed a lot of pronunciation practice, but by the time she first uses it for real she can recite it like it's her address. Her French teacher commented last week on how much her accent has improved from last year's mediocre baseline.)

This spell works and doesn't disrupt if she turns the box over!

Excellent! Obviously that isn't going to work for anything other than a pre-sharpied-on cubical box, but the point is to build up a sense of exactly what she needs to specify in exactly what terms. Next step is putting things in there: can she fill this box with a quantity of non-enchanted socks, pencils, paperback books, and other random bedroom items it could not have previously encompassed?

She sure can. There is twice as much box in this box as there should be.

Aaaand if she dumps it all out and stands well back and pitches a glowing pebble at it? (And goes and gets it when she misses, and pitches it again?)

When she gets it in it lands like a normal object would.

Except for how it's still glowing with magical heatless light, yeah? 

Hmm, where to go from here. Honestly she could probably get some nice wooden boxes and sell what she has, but there are a few directions worth expanding in. She writes up some notes:

* Directions other than depth (too trippy?)

* Expanding a magically durable box: either the safer order or distinctly not that

* Bags

     > various directions

     > transparent plastic bag, just to see what it looks like???

* Nesting 

* Ask authorities about population density

She spends the rest of the night reading what she can of a French geometry textbook she found online.

It continues to glow, yes.

Or it does until she disenchants it, anyway. She's taken to keeping the number of magic objects in her room to a minimum consisting mainly of Endurance Test Rock.

The next night is trivia night, so the ideas she adds to her notes while her classmates aren't looking have to wait.