Snape Let It Slip

Lupin crossed to the door and closed it behind Harry.

'No. Professor Dumbledore managed to convince Fudge that I was trying to save your lives.' He sighed. 'That was the final straw for Severus. I think the loss of the Order of Merlin hit him hard. So he — er — accidentally let slip that I am a werewolf this morning at breakfast.'

— From Harry Potter and the Prisoner of Azkaban

'Miss Parkinson, are all potions instantaneous in effect?'

'No, Professor; there are many, such as slow-acting poisons, that may be administered in small doses, either to disguise the effects, or,' she smirked, 'to lengthen the sufferings of the victim.'

'Quite right — fifteen points to Slytherin. Mr. Crabbe, for what other purpose might one use small doses?'

'Er... '

'For instance, to counteract what recurring nocturnal phenomenon ... that is,' he continued hastily, as Crabbe's chin slid deeper into his neck, 'what thing or things that come out ... every night ... up in the sky... ?'

'Er ... stars?'

'Well done, Crabbe; ten more points to Slytherin. The influence of certain stars or planets may be hindered or advanced by augmenting or decreasing the amount of a potion imbibed. For instance,' (Snape's black eyes narrowed), 'a Wolfsbane Potion, such as is used in counteracting lycanthropy — that is werewolfism, Mr. Goyle — should be drunk in increasingly large quantities as one approaches the time of the full moon. Of course, one must be careful. Why, Mr. Malfoy?'

'Because, sir,' said Draco, with the utmost devotion and sincerity shining in his eyes, 'wolfsbane — also known as monkshood or aconite, as I believe you told us once, sir — is a dangerous poison.'

'Quite right, Mr. Malfoy — twenty points to Slytherin. You may notice its ill-effects on Professor Lup—'

'Ooooh, sir!' (A chorus of voices.) 'Professor who, sir? Professor Lupin? Is Lupin a werewolf then, sir?'

'SILENCE. I did not say Professor Lupin was a werewolf. I merely pointed out the effects of certain potions in conjunction with certain celestial phenomena, such as the full moon. For the rest, you must use your own intelligence — which, though perhaps as your House-Master I should not say it,' (and the corners of his thin mouth crinkled slightly upward) 'I have always regarded as more than equal to ... shall one say, the niceties of school life?' (A very soft and very malicious chuckle ran round the room.) 'You will be eager for your morning conversations over the buttered toast and jam with your friends in other Houses. I might particularly remind you to be especially friendly to your classmates in Gryffindor House. Let us go up to breakfast.'