Wivenhoe (WIV-en-hoh), n. The cry of alacrity with which a sprightly octogenarian breaks the ice on the lake when going for a swim on Christmas Eve.

As the wintry dawn broke over Hogwarts School, the great, oaken door creaked open, and Albus Dumbledore strode outside and inhaled a deep breath of frosty air.

"Lovely," he murmured to himself. "Much better conditions than last year. Sibyll will be terribly disappointed." (Professor Trelawney had predicted raging blizzards throughout the Christmas holidays.) "Well, let's get on with it."

He raised his wand, pointed it at the lake, and cried, with great zest, "Glacifractus!" A jet of orange light shot from the end of his wand to the Black Lake, splintering the sheet of ice that covered it.

"At it again, are we, Headmaster?" said a hoarsely sardonic voice behind him. Dumbledore turned; standing behind him in the entrance hall, still clad in their dressing gowns, were Professor Snape and the new Arithmancy mistress, Theano Vector.

"Ah!" said Dumbledore brightly. "Morning, Severus, Theano. Care to join me for a little dip?"

His two subordinates exchanged glances. "Headmaster," said Snape, in much the same tone that he used when a student confused mistletoe juice and rattlesnake blood, "it is 24 December, and we are at latitude 59° N. Even the giant squid is not swimming in the Lake this morning."

"Well, that's because it's a mollusc," said Dumbledore. "The poor thing can't help itself. But you, Severus –" (and he clapped the Potions master on the shoulder) "– are a healthy mammal with a perfectly functional self-heating system. There's no need for you to hibernate through the winter. Seize the day! Live life to its fullest!"

"Is he always like this on Christmas Eve morning?" said Vector.

Snape nodded solemnly. "We've already lost one Defence against the Dark Arts teacher to this little hobby of his," he said. "Poor Meridia had no conception what the touch of -20° water would do to someone with lamia blood."

"Yes, it was rather foolish of her," Dumbledore agreed. "But the Healers tell me that she's expected to regain consciousness any day now. And, in any event, her unfortunate limitations oughtn't to impede the rest of us any more than the squid's. Come now, you only live once."

"Yes, I do," Snape agreed. "And therefore I have no intention of wasting precious moments of that life in self-inflicted hypothermia. I shall spend this Christmas-Eve morning, as I have spent all Christmas-Eve mornings since I came to this castle, safely in my office with a cup of hot tea and a house-elf preparing a roaring fire. I feel this to be my duty; if the Head of Slytherin House doesn't set an example of sanity, most of the students at this school will never learn what it looks like." And he bowed to the Headmaster, bowed again to Professor Vector, and strode briskly from the entrance hall.

Dumbledore sighed. "A fine mind, Severus's," he said. "One only wishes it was a bit more open to new experiences." He turned to the lone remaining member of his staff. "Well, Theano, what say you?"

Vector hesitated. Her common sense was telling her to join Snape in declining the Headmaster's invitation, but, as was often the case with Theano Vector, her common sense was far from being the only motivation in play. For one thing, having only been on the Hogwarts faculty for four months, she was still in enough awe of Dumbledore to assume that anything he enjoyed doing must be a fitting occupation for one who aspired to his heights of wisdom. For another, she had, like many pure academics, a tendency to worry that she was letting the realities of life slip past while she analysed her theories of it, which made her uniquely vulnerable to the Headmaster's carpe-diem argument.

"Well," she said slowly, "I suppose it couldn't hurt anything to try… perhaps…" She hesitated a second longer, and then took (so to speak) the plunge. "Yes, all right," she said. "Just this once."

Dumbledore beamed. "That's the spirit," he said, with such warm admiration in his voice that Vector couldn't help glowing with satisfaction. Let Severus have his tea and his fire, she thought. I'm going out to savour the fierce tang of existence with the greatest wizard in the world. How many people can say that?

She turned toward the double doors of the Great Hall and broke into an eager run. She was halfway across the floor of the entrance hall when Dumbledore's voice reached her ears. "Taking a short cut, Theano?" he said, sounding puzzled.

Vector halted, and glanced over her shoulder. "Pardon?"

"Well, it's a small point, of course," said Dumbledore, "but, in my experience, it's a trifle easier to get to the Lake by heading towards it."

Vector blinked for a moment, then realised. "Oh, I see," she said with a laugh. "No, I'm just popping up to my office to get a bathing-costume. I suppose you already have yours with you, but I hadn't planned on doing this, you see."

"Ah." Dumbledore nodded reflectively. "Yes, of course. A bathing-costume. Quite."

But there was something about the way he said it that introduced a sudden, unsettling doubt into Vector's mind. "You do have yours with you, Headmaster… don't you?"

"To the best of my knowledge, I don't even own one," said Dumbledore. "I believe I lent my last one to your predecessor when he went to that conference in Aruba. Hippasus was always dreadfully bad at returning things."

This put a new light on the matter. Vector licked her lips (which had suddenly gone dry) and ran a hand through her hair. "Er… actually, Headmaster, you know, I still have a number of mid-term exams to grade," she said. "It might be better… not that I shouldn't love to, of course, don't get that impression, but… well, you know, duty before pleasure, don't you think?"

"Oh, certainly," said Dumbledore with perfect gravity. (Vector thought she saw a tell-tale twinkle in his eye, but at that distance it was difficult to be sure.) "Another time, perhaps?"

"Oh, yes, of course," said Vector. "Well, er… good luck."

And, without waiting for a reply, she turned and fled for her office, reflecting, as she went, how fortunate it was that the board of governors supplied each professor with a private Pensieve. She had never before realised how tremendously important it could be, sometimes, to be able to siphon certain thoughts out of one's mind.