"The dust has dust," Hermione muttered to herself, threw open the tiny window, and began to clean.  She hadn't wanted to learn any of Molly's seemingly endless repertoire of cleaning charms.  They felt oppressive, or, rather, it felt oppressive that she wanted to teach them to Hermione and Ginny but didn't seem inclined to waste her time bothering with Ron or Percy.  She'd been too polite to tell the woman no, however, and so she'd learned.