"But Potter seems to have a lot to say on the subject,” said Snape, pointing suddenly at the back of the room, his black eyes fixed on Harry. “Let us ask Potter how we would tell the difference between an Inferius and a ghost.”
The whole class looked around at Harry, who hastily tried to recall what Dumbledore had told him the night that they had gone to visit Slughorn. “Er — well — ghosts are transparent —” he said.
“Oh, very good,” interrupted Snape, his lip curling. “Yes, it in easy to see that nearly six years of magical education have not been wasted on you, Potter. ‘Ghosts are transparent.’"
- JK Rowling; Harry Potter and the Half-Blood Prince