I hate teenagers.

Lips pulled back from his teeth, Severus scrubbed at the potions laboratory's heavy stone countertops.

So much.

It was thick and tacky, each large spatter clinging stubbornly wherever it had landed after the initial explosion. It was completely resistant to scouring charms, and none of the solvents he'd tested had had an appreciable effect. It was dark evergreen and smelled like pine, except where it was deep red scented with cinnamon, and it was all over his classroom.

A fat drop of icy water landed squarely on his head from the icicles dangling above.

Severus snarled.

Detention had yet to have any appreciable effect on the Weasley twins, but it just might quell the group of second-year Ravenclaws responsible for the worse part of the mess. If not for this year's annual foolishness, I'd be long done by now.

But instead, he'd been chaperoning hormone-ridden students and trapped into dancing with Rolanda of all people, while the mess splattered across his dungeons had congealed into tar and the hundreds of icicles fused to the ceiling melted, cracked, and smashed to the floor. His sanctuary was a Christmas-themed warzone.

Arm burning, fingers clenched tight around a wire scouring brush, Severus leant the whole of his weight against the scented glop and growled.

Snap!

Pain smacked into his side as the brush unexpectedly gave way, spilling him unceremoniously against the edge of the stone countertop. Panting, he stared in disbelief at the brush's wooden handle, snapped clean in two. This is not happening.

Nearly midnight, Christmas Eve, and –

He could hear something.

Beyond the noise of his own furious, heaving lungs, a sound drifted through the dungeon door – propped open to dissipate the nearly-overwhelming odor of pine.

It was a voice, but he couldn't . . . quite . . .

A student out after curfew.

Rage blanking out everything else, Severus burst out of the dungeons, pausing just long enough to pick the most likely direction based on the faint echoes.

Halfway down the hallway, the strangely haunting murmur resolved into words.

" – Lord of all the Earth sent Angel choirs instead –"

It was a girl, and she was singing.

Low, and somehow mournful for what must be a carol of some sort. Severus followed the stairs upward to the main level of the castle, emerging into a back hallway that was completely empty of anyone, singing or not.

"Before their light, the stars grew dim –"

It was louder. Severus kept his feet quiet against the flagstones; he was going the right way. And he was going to heap detention on whatever idiot was stupid enough to break curfew and then sing about it. Even if he'd never heard the song before.

" – your King is born –"

The music faded abruptly as Severus sped past a hallway lit only by tall windows, and he quickly backtracked.

"In Excelsis Gloria . . ."

There.

"You!" Severus snapped, registering with satisfaction the way the girl leaning against the windowsill started in shock. "What are you doing out after –" She turned to face him, snow-bright moonlight illuminating her face, and his voice died. "Curfew," Severus finished weakly. "Susan."

"Severus." Blue eyes took him in, skimming over lank, dripping hair, past disheveled robes with sleeves rolled-up to reveal hands chapped and reddened with scrubbing, and down to the ratty slippers on his feet. Which were embroidered with glinting Christmas trees, because Albus had a fondness for both sentimental touches on practical gifts, and the absurd. "I can see you're getting into the holiday spirit."

Anyone else would have been mocking; she was only gently teasing.

His tension slipped down a notch, entirely involuntarily. "It is the season of giving," he agreed, sneering elegantly.

Her laughter reminded him of low-chiming bells. "I don't think detention is quite what they had in mind."

The silence between them was comfortable; Severus leant against the freezing windowsill opposite Susan, and made a show of looking outside. The moon was full and radiant, peeking eagerly through clouds swirling with the promise of snow. "How's your family?" he said awkwardly, the holiday and company making him try for social niceties on which he rarely wasted time. Why aren't you with them? was what he wouldn't ask, though he wanted to.

Susan stiffened. As usual, she heard what he meant rather than what he said, and for once, looking at shoulders strung tight under a thick fall of black hair, Severus wasn't glad for it. "Lucy fell asleep waiting for the snow, but Ed will probably wake her up soon. Peter's around somewhere."

"Ah." Severus cleared his throat uncomfortably, shifting on his feet.

"And you've been cleaning," she continued, smoothing over his discomfort.

"Mm," Severus nodded. "I – heard you. Singing," he blurted out.

She just nodded, unconcerned; but blue eyes gazed solidly out onto the castle lawn.

"It was nice," he heard himself say, and froze, horrified. Where did that come from? He kept his own eyes firmly fixed out the window, taking in the cold glint of moonlight off the distant lake.

When he darted a glance sideways, Susan's cheeks had pinked. "Thank you."

After a few false starts, Severus got out, "I didn't mean to make you stop."

"Oh." Susan had that gift, though – of knowing him well enough to somehow know what his intent was, though he could never speak it. Comfort crept back into the silence between them, and with it, came a joyful noise.

She sang.

He closed his eyes, breathed deeply, and listened.

"The earliest moon of wintertime is not so round and fair, as was the ring of glory on the helpless infant there. The chiefs from far before him knelt, with gifts of fox and beaver pelt – Jesus, your King is born! Jesus is born! In Excelsis Gloria!"

Beyond frost-limned glass, the first flakes drifted down out of a starry night sky.