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"Mr. Weasley said we're trying to seem like Muggles," Dean said, noticing how Harry was looking around. "I think this is what happens when most of them have never done Muggle Studies and the rest of them did it back in the nineteen twenties or something."

"Hey, we're doing all right," Fred protested. "Aren't we?"

George nodded. "Of course. After all, if we were wrong we'd know."

"That's not how… um… knowledge works," Hermione said, after a pause of three or four seconds to make sure she knew just how she was going to phrase it.

"It is," Ron countered. "In History of Magic there's lots of questions where I think 'I don't know that'. You can know you don't know something."

"I know, but you can also not know you don't know something," Hermione replied. "If you have the wrong idea about something, you can be really sure of it and still be wrong – like Muggles are about magic, because they don't know."

"That is a good point," Ron admitted, after thinking about it a bit. "Oh, so maybe Fred and George aren't dressed right for Muggles."

Harry thought they didn't look all that bad, really, for wizards dressed as Muggles. If he'd seen them on the streets of London he would have just thought they were teenagers, and teenagers could wear almost anything – if you didn't recognize it, according to the books he'd read, that just meant they were rebelling against something.

"When in doubt, wear a football shirt," Dean advised. "That's what I'd say."

http://www.fanfiction.net/s/13230340/52/