Harry Potter and the Methods of Rationality

"Aw," said Hermione. "It sounds like Mr. Crabbe is taking a little nap."

Like two shoals of fish, the armies swam along.

Neville Longbottom kicked his feet in slow, measured motions. Diving, always diving in whatever direction you happened to be moving. You wanted to show the enemy the smallest profile, present them with your head or your feet. So you were always diving, downward and head-first, and the enemy was always down.

Like every Chaos Legionnaire in the army, Neville's head was constantly rotating as he swam, looking up, down, around, to every side. Not just watching for Sunshine Soldiers, but watching for any sign that a Chaos Legionnaire had drawn their wand and was about to betray them. Usually traitors waited until the confusion of battle to make their move, but that early gong had put them all on guard.

...the truth was, Neville was feeling sad about that. In November he'd been a soldier in a united army, all of them pulling together and helping each other, and now they were all watching each other constantly for the first signs of betrayal. It might have been more fun for General Chaos, but it wasn't nearly as much fun for Neville.

The direction formerly known as 'up' was getting steadily brighter, as they came closer to the surface and Sunshine.

"Wands out," said General Chaos.

Neville's squad drew their wands, pointing them straight ahead toward the enemy, as their heads scanned around more rapidly. If there were Sunny traitors, the time was approaching for them to strike.

The other shoal of fish, Dragon Army, was doing the same thing.

"Now!" shouted the distant voice of the Dragon General.

"Now!" shouted General Chaos.

"For Sunshine!" shouted all the soldiers in both armies, and charged downward.

"What?" said Minerva involuntarily as she watched the screens from next to the lake, a cry echoed in many other places; all of Hogwarts was watching this battle as they had watched the first.

Professor Quirrell was laughing dryly. "I warned you, Headmaster. It is impossible to have rules without Mr. Potter exploiting them."

For long precious seconds, as the forty-seven soldiers charged her own seventeen, Hermione's mind went blank.

Why...

Then it all snapped into place.

Every time a soldier originally from Sunshine got shot by someone crying the name of Sunshine, she would lose a Quirrell point. When two Sunshine Soldiers were shot by either army, both enemy armies would be two points closer to overtaking her, it was the same gain only shared. And if anyone shot another soldier not in the name of Sunshine, that gong wouldn't get lost in the confusion...

Hermione was suddenly very glad that Zabini hadn't gone with the obvious plan of starting trouble between the other two armies while they attacked Sunshine.

It was still disheartening, though, that sense of your chances closing down, of hope being taken away.

Most of Hermione's soldiers were still looking confused, but some had expressions of dawning horror as they got it.

"It's all right," Susan Bones said firmly. Heads turned to look at the Sunshine Captain. "Our job is the same, to take as many of them with us as we can. And remember, Zabini took away all the spies! We don't have to stay on the lookout like they do!" The girl was smiling defiantly, provoking answering smiles from many of the other soldiers, even from Hermione herself. "It can be like it was in November. We just have to keep our heads high, fight our best, and trust each other -"

Daphne shot her.

"Blood for the blood god!" shrieked Neville of Chaos, though since he was underwater it came out more like 'Blubbled for the blubbled glub!'

Captain Weasley spun and raised his wand toward Neville and fired. But Neville was swimming downward toward him, wand pointed straight ahead, and that meant the Simple Shield could shelter Neville's entire profile; if anyone shot him now, it wasn't going to be Sunny Ron.

A grimly determined look came over Captain Weasley's face, and he arrowed straight up toward Neville, mouthing the word Contego, though the shield wasn't visible in the water.

The two enemy champions shot toward each other like arrows released from bows, each aimed to split the other down the middle. They had dueled many times before, but this time would pay for all.

(Far away by the lakeside, a hundred breaths were held.)

"Rainbows and unicorns!" roared the Sunshine Captain.

"The Black Goat with a thousand young!"

"Do your homework!"

Closer and yet closer, the two champions charged, neither willing to swerve, the first person to turn would present a vulnerable broadside and get shot, though if neither lost their nerve they would crash right into each other...

Falling straight down as the enemy rose straight up to meet him, hammer descending to meet anvil in a path neither was willing to leave...

"Special attack, Chaotic Twist!"

Neville saw the look of horror on Captain Weasley's face as the Hover Charm caught him. They'd tested it before the battle had started; and just as Harry had suspected, Wingardium Leviosa became a whole new sort of weapon once everyone was swimming underwater.

"Curse you, Longbottom!" shrieked Ron Weasley, "Can't you ever fight