The dementors were retreating, although not before making it clear that most patronuses did no good against them. In fact, it seemed to feed them. So the question to Mo, with his big ol' Ravenclaw brain, was what was different about Vinteren's patronus? Why did it work, when no one else's did?

Mo found his wand was out in his hand, which was a comfort. At least he tried to fight back, even if he'd never produced a corporeal patronus. He holstered his wand and scooted toward the big scary professor.

"Professor? Why don't they eat your patronus too?"