"And what the ruddy hell are dementors?"
"They guard the wizard prison, Azkaban," said Aunt Petunia.
Two seconds' ringing silence followed these words and then Aunt Petunia clapped her hand over her mouth as though she had let slip a disgusting swear word. Uncle Vernon was goggling at her. Harry’s brain reeled. Mrs. Figg was one thing — but Aunt Petunia?
"How d'you know that?" he asked her, astonished.

Harry Potter and the Order of the Phoenix