The Founder

"My Hufflepuff cup," she nodded proudly. "It has an Undetectable Extension charm on it. Yes, I know," she said when Sirius leaned over and saw a broomstick tucked neatly inside the cup. "It's bigger on the inside than on the outside." — in which a muggle Sirius meets a time-travelling Founder. Inspired by JKR and Doctor Who. AU, of course.