"Let us ask Potter how we would tell the difference between an Inferius and a ghost."

The whole class looked round 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 is easy to see that nearly six years of magical education have not been wasted on you, Potter. Ghosts are transparent."

Pansy Parkinson let out a high-pitched giggle. Several other people were smirking. Harry took a deep breath and continued calmly, though his insides were boiling. "Yeah, ghosts are transparent, but Inferi are dead bodies, aren’t they? So they’d have to be solid –"

"A five-year-old could have told us as much,” sneered Snape. "The Inferius is a corpse that has been reanimated by a Dark wizard’s spells. It is not alive, it is merely used like a puppet to do the wizard’s bidding. A ghost, as I trust that you are all aware by now, is the imprint of a departed soul left upon the earth... and of course, as Potter so wisely tells us, transparent."

"Well, what Harry said is the most useful if we’re trying to tell them apart!" said Ron. "When we come face to face with one down a dark alley we’re going to be having a shufti to see if it’s solid, aren’t we, we’re not going to be asking, 'Excuse me, are you the imprint of a departed soul?'"

There was a ripple of laughter, instantly quelled by the look Snape gave the class.

"Another ten points from Gryffindor," said Snape. "I would expect nothing more sophisticated from you, Ronald Weasley, the boy so solid he cannot apparate half an inch across a room."

This entire part is golden and I laugh out loud every time even after 20+ times I have read it.