SELENA WISHED THEY COULD'VE put the mechanical spider on a leash. It scuttled along the tunnels so fast, most of time she couldn't even see it. If it hadn't been for Tyson's and Grover's excellent hearing, they never would've known which way it was going.

They ran down a marble tunnel, then dashed to the left and almost fell into an abyss. Tyson grabbed Percy and hauled him back before he could fall. The tunnel continued in front of them, but there was no floor for about a hundred feet, just gaping darkness and a series of iron rungs in the ceiling. The mechanical spider was about halfway across, swinging from bar to bar by shooting out metal web fiber.

"Monkey bars," Annabeth said. "I'm great at these."

She leaped onto the first rung and started swinging her way across. She was scared of tiny spiders, but not of plummeting to her death from a set of monkey bars. Go figure.

Annabeth got to the opposite side and ran after the spider. Percy followed then Selena. When they got across, they looked back and saw Tyson giving Grover a piggyback ride (or was it a goatyback ride?). The big guy made it across in three swings, which was a good thing since, just as he landed, the last iron bar ripped free under his weight.

They kept moving and passed a skeleton crumpled in the tunnel. It work the remains of a dress shirt, slacks, and a tie. The spider didn't slow down. Percy slipped on a pile of wood scraps, but when he shined a light on them he realized they were pencils—hundreds of them, all broken in half.

The tunnel opened up onto a large room. A blazing light hit them. Once her eyes adjusted, the first thing Selena noticed were the skeletons. Dozens littered the floor around them. Some were old and bleached white. Others were more recent and a lot grosser. They didn't smell quite as bad as Geryon's stables, but almost.

Then Selena saw the monster. She stood on a glittery dais on the opposite side of the room. She had the body of a huge lion and the head of a woman. She would've been pretty, but her hair was tied back in a tight bun and she wore too much makeup, so she kind of reminded me of my third-grade choir teacher. She had a blue ribbon badge pinned to her chest that took Selena a moment to read: THIS MONSTER HAS BEEN RATED EXEMPLARY!

Tyson whimpered. "Sphinx."

Annabeth started forward, but the Sphinx roared, showing fangs in her otherwise human face. Bars came down on both tunnel exits, behind them and in front.

Immediately the monster's snarl turned into a brilliant smile.

"Welcome, lucky contestants!" she announced. "Get ready to play...ANSWER THAT RIDDLE!"

Canned applause blasted from the ceiling, as if there were invisible loudspeakers. Spotlights swept across the room and reflected off the dais, throwing disco glitter over the skeletons on the floor.

"Fabulous prizes!" the Sphinx said. "Pass the test, and you get to advance! Fail, and I get to eat you! Who will be our contestant?"

Annabeth stepped forward "I've got this," she whispered. "I know what she's going to ask."

Selena didn't argue too hard. She didn't want Annabeth getting devoured by a monster, but Selena figured if the Sphinx was going to ask riddles, Annabeth was the best one of them to try.

She stepped forward to the contestant's podium, which had a skeleton in a school uniform hunched over it. She pushed the skeleton out of the way, and it clattered to the floor.

"Sorry," Annabeth told it.

"Welcome, Annabeth Chase!" the monster cried, though Annabeth hadn't said her name. "Are you ready for your test?"

"Yes," she said. "Ask your riddle."

"Twenty riddles, actually!" the Sphinx said gleefully.

"What? But back in the old days—"

"Oh, we've raised our standards! To pass, you must show proficiency in all twenty. Isn't that great?"

Selena gulped nervously. Luckily Annabeth was the child of Athena.

Applause switched on and off like somebody turning a faucet.

Annabeth glanced at them nervously. Selena gave her an encouraging nod.

"Okay," she told the Sphinx. "I'm ready."

A drumroll sounded from above. The Sphinx's eyes glittered with excitement. "What...is the capital of Bulgaria?"

Annabeth frowned but Selena knew that Annabeth knew the answer.

"Sofia," she said, "but—"

"Correct!" More canned applause. The Sphinx smiled so widely her fangs showed. "Please be sure to mark your answer clearly on your test sheet with a number 2 pencil."

"What?" Annabeth looked mystified. Then a test booklet appeared on the podium in front of her, along with a sharpened pencil.

"Make sure you bubble each answer clearly and stay inside the circle," the Sphinx said. "If you have to erase, erase completely or the machine will not be able to read your answers."

"What machine?" Annabeth asked.

The Sphinx pointed with her paw. Over by the spotlight was a bronze box with a bunch of gears and levers and a big Greek letter Ȇta on the side, the mark of Hephaestus.

"Now," said the Sphinx, "next question—"

"Wait a second," Annabeth protested. "What about 'What walks on four legs in the morning'?"

"I beg your pardon?" the Sphinx said, clearly annoyed now.

"The riddle about the man. He walks on four legs in the morning, like a baby, two legs in the afternoon, like an adult, and three legs in the evening, as an old man with a cane. That's the riddle you used to ask."

"Exactly why we changed the test!" the Sphinx exclaimed. "You already knew the answer. Now second question, what is the square root of sixteen?"

"Four," Annabeth said, "but—"

"Correct! Which U.S. president signed the Emancipation Proclamation?"

"Abraham Lincoln, but—"

"Correct! Riddle number four. How much—"

"Hold up!" Annabeth shouted.

Selena wanted to tell her to stop complaining. She was doing great! She should just answer the questions so they could leave.

"These aren't riddles," Annabeth said.

"What do you mean?" the sphinx snapped. "Of course they are. This test material is specially designed—"

"It's just a bunch of dumb, random facts," Annabeth insisted. "Riddles are supposed to make you think."

"Think?" The Sphinx frowned. "How am I supposed to test whether you can think? That's ridiculous! Now, how much force is required—"

"Stop!" Annabeth insisted. "This is a stupid test."

"Um, Annabeth," Grover cut in nervously. "Maybe you should just, you know, finish first and complain later?"

Selena raised her hand, "I agree."

"I'm a child of Athena," she insisted. "And this is an insult to my intelligence. I won't answer these questions."

"Oh my Gods, Annie." Selena sighed,

The spotlights glared. The Sphinx's eyes glittered pure black, "Why then, my dear," the monster said calmly. "If you won't pass, you fail. And since we can't allow any children to be held back, you'll be EATEN!"

The Sphinx bared her claws, which gleamed like stainless steel. She pounced at the podium.

"No!" Tyson charged. He hates it when people threaten Annabeth, but I couldn't believe he was being so brave, especially since he'd had such a bad experience with a Sphinx before.

He tackled the Sphinx in midair and they crashed sideways into a pile of bones. This gave Annabeth just enough time to gather her wits and draw her knife. Tyson got up, his shirt clawed to shreds. The Sphinx growled, looking for an opening.

Selena drew her dagger and Percy drew Riptide and the two of them stepped in front of Annabeth.

"Turn invisible," Percy told her.

"I can fight!"

"No!" Selena yelled. "The Sphinx is after you! Let us get it."

As if to prove her point, the Sphinx knocked Tyson aside and tried to charge past them. Grover poked her in the eye with somebody's leg bone. She screeched in pain. Annabeth put on her cap and vanished. The Sphinx pounced right were she'd been standing, but came up with empty paws.

"No fair!" the Sphinx wailed. "Cheater!"

With Annabeth no longer in sight, the Sphinx turned on Percy. He raised his sword, but before he could strike, Tyson ripped the monster's grading machine out of the floor and threw it at the Sphinx's head, ruining her hair bun. It landed in pieces all around her.

"My grading machine!" she cried. "I can't be exemplary without my test scores!"

The bars lifted from the exits. They all dashed for the far tunnel. Selena could only hope Annabeth was doing the same.

The Sphinx started to follow, but Grover raised his reed pipes and began to play. Suddenly the pencils remembered they used to be parts of trees. They collected around the Sphinx's paws, grew roots and branches, and began wrapping around the monster's legs. The Sphinx ripped through them, but it brought them just enough time.

Tyson pulled Grover into the tunnel, and the bars slammed shut behind them.

"Annabeth!" Selena yelled.

"Here!" she said, appearing right next to her. "Keep moving!"

They ran through the dark tunnels, listening to the roar of the Sphinx behind them as she complained about all the tests she would have to grade by hand.