"Fancy seeing you here, Professor McGonagall."
"How did you know it was me?" she asked.
"My dear Professor, I 've never seen a cat sit so stiffly."
"You'd be stiff if you'd been sitting on a brick wall all day," said Professor McGonagall.

Harry Potter and the Philosopher's Stone - Chapter 1 : The Boy Who Lived
See this quote in the Pensive -