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"I must say that I am perplexed as to how you were able to accomplish this." Professor Dumbledore admitted. "Many wizards, myself included, can increase the size of our Patronuses but doing so causes them to break apart. You were somehow able to keep yours intact, do you have any idea how that was done?"

"Yes, Professor. I think I figured it out, with Hermione's help that is." Harry explained. "I sort of figured out for myself that they way Professor Lupin taught me the charm wasn't the best way to do it. What memory you focus on isn't important, you have to make yourself feel a positive emotion. The spell works in two parts: your magic creates the mist that actually drives the Dementors away but your emotion contains and shapes the mist. Hermione compared the mist that the spell creates to a gas and the Patronus itself to the vessel that you put the gas into. If you try and force a lot of gas into a weak container then the container will break or leak, but she says that if you have a really strong container and you force a bunch of gas into it you can force the gas to change into a liquid. It didn't make sense to me, but she said she got the idea from muggle chemistry. She thinks that's what I did, except we were using magic and not gas."

"Interesting... I never would have thought to use muggle science to describe magic. Bright girl, Ms Granger, I still think she should have been a Ravenclaw." Professor Flitwick said with a small laugh. "Your method is a bit different from the accepted practice, although it clearly does seem to have an advantage."

http://www.fanfiction.net/s/5244813/13/