This just struck me. It's un-betaed so please forgive any errors they are all mine.
The Birthday Card
Remus Lupin was eating his breakfast in a far more subdued manner than was normal for him. When the owls arrived, he didn't even look up, not expecting any deliveries.
So, when an owl landed gracefully next to his plate and stole some bacon, Remus jumped making the resident potions master sitting next to him snort in amusement.
Remus took the letter from the owl and it flew away stealing the last of his bacon. Breaking the seal Remus withdrew a card, a birthday card to be exact.
The card was handmade; on the cover was an intricately drawn picture of Remus himself sitting at his desk, holding a quill. The picture was drawn from the point of view of the back of the classroom; the artist had drawn the backs of the student's heads each bent over their books while Remus had a smile on his face looking incredibly happy.
Above the picture, in beautiful script, were the words.
"When you get older, there are two things you must remember..."
Opening the card, there was a plane sheet of paper with the words.
"Never trust a fart..."
Remus sniggered, and turned to the last page. What he saw made him gasp.
Another picture looked back at him. This picture was, drawn from Remus's point of view, looking down. Under his desk was another man, enthusiastically sucking Remus's cock while fisting his own. This drawing was moving.
Underneath were the words.
"...And never waste an erection."
The drawing Remus came and so did the man pleasuring him. When the man sat back on his heals, Remus gasped aloud. Looking back at him was a perfectly drawn Severus Snape.
Remus's head snapped up to look at the real Severus sitting calmly next to him sipping his tea, "Do close your mouth, Lupin. You have a class in ten minutes, don't you?"
Remus nodded dumbly.
"Good. It is good advice after all, and I believe I can help with one of them."
A feral smile spread across Remus's face, "You realise, that a werewolf stamina..." Remus started.
"Will have you hard frequently, I am aware of that fact yes," Severus finished his tea and stood. "I never start something I can't finish."
Remus pushed his plate away and stood too, "It wasn't a no, Severus, merely a warning, and a promise. You are two months older than me after all."
"A promise of what, may I ask?" Severus asked as they walked towards Remus' classroom, Severus discreetly casting a notice-me-not charm on himself.
"A promise, that after you've made that stunning card come true I'm going to bend you over my desk and fuck you through it." Remus told him opening his classroom door letting them both in.
"A promise I expect you to keep," Severus told him making himself comfortable under the desk.
"I never make a promise I don't intend to keep," Remus informed him undoing his trousers and making himself comfortable in his chair.
"Happy Birthday Remus."
"Come in class and take your seats. Today will not be a practical lesson. We will be learning about..."