This was what confirmed Eliezer's skill as a writer in my mind. He resisted the (typical nerdish) impulse to vomit out pages of obsessively detailed explanations, instead leading the reader on with tantalising hints spaced far apart. It probably accounts for a lot of the book's notorious addictiveness.

From the very end of Chapter 4:

"That's the spirit! And does a 'mokeskin pouch' do what I think it does?"

"It can't do as much as a trunk," McGonagall said reluctantly, "but a mokeskin pouch with a Retrieval Charm and Undetectable Extension Charm can hold a number of items until they are called forth by the one who emplaced them."

"Yes, I definitely need one of those too. It's like the super beltpack of ultimate awesomeness! Batman's utility belt of holding! Never mind a swiss army knife, you could just carry a whole tool set in there! Or other magic items! Or books! I could have the top three books I was reading on me at all times, and just pull one out anywhere! I'll never have to waste another minute of my life! What do you say, Professor McGonagall? It's in the best of all possible causes."

"Fine. You may add another ten Galleons."

Griphook was favoring Harry with a gaze of frank respect, possibly even outright admiration.

"And a little spending money, like you mentioned earlier. I think I can remember seeing one or two other things I might want to store in that pouch."

"Don't push it, Mr. Potter."

"But oh, Professor McGonagall, why rain on my parade? Surely this is a happy day, when I discover all things wizarding for the first time! Why act the part of the grumpy grownup when instead you could smile and remember your own innocent childhood, watching the look of delight upon my young face as I buy a few toys using an insignificant fraction of the wealth that I earned by defeating the most terrible wizard Britain has ever known, not that I'm accusing you of being ungrateful or anything, but still, what are a few toys compared to that?"

"You," McGonagall growled. There was a look on her face so fearsome and terrible that Harry squeaked and stepped back, knocking over a whole pile of gold coins with a great jingling noise and sprawling backward into a heap of money. Griphook sighed and put a palm over his face. "I would be doing a great service to wizarding Britain, Mr. Potter, and perhaps the entire world, if I locked you in this vault and left you here."

And they left without any more trouble.