Pendoggett Farm was dark, noisy, and full of cages. The “farm” part of its name had left Hannah expecting green pastures and creatures roaming free behind wire netting – but this was nothing like that. The place resembled the aquarium Hannah had visited on a school trip to Baltimore, only quieter, colder, emptier. The kind of empty that made Hannah feel like something bad was going to happen.

An exciting kind of bad, she told herself. This wasn’t another one of Warren’s lectures. This was real.

Hannah squinted through glass at a pair of dust spirits. They glowered at her from between mounds of sand, their red eyes fierce and sinister slits.

“They don’t look too happy back there, do they?”

Harry had appeared behind her. He frowned at the dust spirits, his lips pursed. They frowned back, grinning; Harry started and stepped away.

“No,” Hannah agreed. “But they’re evil, aren’t they? They’re probably never happy unless they’re hurting people. Like Warren said, this is the best place for them.”

They moved on, stopping every few feet to examine the way various creatures reacted to being trapped in five-by-ten-foot Plexiglas cages. Two brownies – rare finds outside of Scotland – had been placed side-by-side. One was given porridge on a regular basis and the other was not. Hannah marveled at how they could even be considered the same creature – the yowling, spitting, snarling ball of fur to the left was nothing like its neighbor, who was happily sweeping up a dust bunny it had found in the corner of its cage.

A few steps onward, they spotted a vampire, curled up in its bat form and hanging from the ceiling of its enormous enclosure, which had been brightly lit from all sides with ultraviolet lightbulbs. A plaque told them that the vampire had become humanoid only once during its captivity, and that was to murder a large rat that had managed to burrow into its space. Connor tapped on the glass and tried to tempt it with a scratch he’d received in P.E. a few days before. The vampire yawned and wrapped its wings more tightly around itself.

Hannah had just reached the cage of the hedley kow, which was mooing in a way that looked fascinatingly agitated, when Warren called the class back to the front of the room. He was standing beside a hard-faced woman in a gray uniform. Something about her made Hannah’s stomach twist uncomfortably.

“That’s all the time we have to explore today,” Warren said. “If you’re curious about any of the creatures you saw in the last twenty minutes, I’d encourage you to ask your parents about them when you get home. For now, I’m putting you in the capable hands of Candice, who works in the eidolon wing.”

The woman – Candice – nodded, silencing the chatter of Hannah’s class. “Warren tells me you haven’t studied eidolons yet,” she said. “Could I see a show of hands from everyone who knows what they are and what they do?”

Min raised a hand; so did Jeremy Pryce and a girl who sat in front of Hannah in pre-algebra. But they were the only people who did.

“That’s pretty typical,” said Candice. “Most people don’t start noticing them until they’re about your age. A lot of adults also tend to be afraid of them – unnecessarily, as I’ll explain, but that fear is there. And it’s been my experience that people don’t like to talk about the things they’re afraid of.” She cleared her throat. “For those of you who do know about eidolons, who told you?”

“My dad,” said Min. “He had a problem with them back when I was little.”

“Same,” said Jeremy Pryce, “except it was my mom.”

“I saw one,” said the girl in Hannah’s math class. “Walking home after school one day. It followed me for a while.”

“Again, that’s typical,” said Candice. “At your age, it’s usually only people with direct experience who know very much about eidolons. For now, I’d like you to put whatever knowledge you have in the back of your mind and follow me to the eidolon wing.”

She took them through a door behind the brownie cages and into a dim, dank-smelling hallway. For the second time, a sharp, unexpected sense of discomfort worked its way into Hannah’s stomach – but she told herself this was a good thing. It was supposed to be scary.

After a few minutes, they reached an enormous steel door with three padlocks holding it shut. Candice used a set of keys that were dangling around her neck and let them in.

At first glance, the room appeared pitch black. Even so, Hannah could tell it was much larger than the room they’d started out in, because of the way her classmates’ voices echoed as they whispered to each other. As her eyes adjusted to the darkness, she noticed what looked like a vast, glinting cloud hovering just below the ceiling.

“There,” said Candice, pointing at it. “Those are eidolons.”

As though it was following her finger, the cloud suddenly dissipated. Individual wisps of it swooped downward to hover just above the class’s heads. A few of Hannah’s classmates yelped and ducked. Hannah crossed her arms and tried to examine the eidolons more closely. They didn’t look like any living creature she’d ever seen before. They didn’t even have faces.

“Now,” said Candice calmly, “as you can see, eidolons are pretty visible in this kind of darkness. However, most of them don’t live locked away like this. Most of the eidolons in the world are floating around outside – maybe above your school, or your neighborhood, or your local grocery store. Anywhere and everywhere.”

Ira raised a hand. “Then how come I’ve never seen one before?”

“Usually, they’re translucent – hard to spot,” said Candice. “But they darken as they get closer to people. They’re close enough to us now that they really should be darker, except that we keep them weak here at Pendoggett, which forces them to stay a little ways away. Although they have sensed that we’re here.”

They swarmed nearer, making wavy, excited movements. Hannah stared, fascinated. It was like seeing smoke dance.

“I’m going to pull them down in a minute,” Candice continued. “There’s only one way to do that, and most people do it accidentally. Eidolons are attracted by only one thing, and that’s the chemicals your body releases when you’re afraid of something. Pheromones – but that’s science. You don’t have to know science to know eidolons. Wait and watch.”

She closed her eyes and squeezed them shut.

For a moment, very little happened. The cloud of eidolons lowered a little, hovered, then lowered again. But as Candice’s eyes stayed closed and her hands clenched into fists, the cloud’s movements changed.

If before the eidolons looked like smoke, they now resembled a heavy, dark arrow. With what looked like the thickest, most corporeal eidolon at the forefront, the cloud rocketed towards Candice, engulfing her shoulders and chest. The darkness of the eidolons made her look sinister, like a head hanging in midair above a waist and a pair of legs, her upper body entirely absent.

Candice opened her eyes, unclenched her fists, and smiled. The eidolons swirled around her, like leaves in a breeze.

“There you have it,” she said. “That’s what eidolons do. That’s all eidolons can do. If an eidolon’s looking for something to feed off of, and you have the bad luck to be walking around beneath it, and you also have the bad luck to be worrying about something, then there’s a good chance it’s going to come hang out with you for a while, just like these are doing. What I did, just there? I had to think hard about something I was worried about – devote all my attention to it – make it the most important thing on my mind. Otherwise, the eidolons were going to leave me pretty much alone.”

Hannah gazed at the swaying cluster of eidolons. Something tugged at the edges of her memory, although she couldn’t for the life of her figure out what it was.

“What were you worrying about?”

Candice shook her head. “I can’t tell you. Talking to people about your worries – being open about them – tends to make them dissipate faster. If I ever got to a point where I could do that, I could lose my job. I couldn’t give demonstrations like this anymore.”

Aimee raised a hand. “You said a lot of grown-ups are afraid of eidolons. If they only sort of… drift on you, why are they so scared? They don’t really do anything, right?”

Candice gave an odd-sounding sigh. “I think it can be hard to explain. You guys are twelve, right? Thirteen? People tend to understand better after they’ve finished high school. The thing is… you can get caught in these cycles. One day you’re just doing your everyday thing, and then something happens to make you worry. And then maybe an eidolon finds you, feeds off that worry.

“The best thing to do, of course – the most effective thing – is to ignore the eidolon. To just keep going about your business. But because you can see it… because there’s a visual reminder of what you’re afraid of… one that follows you wherever you go… it’s much harder to calm yourself down. And so you worry more. And the eidolon feeds off of that new worry, and maybe invites some friends.”

“And then you might start to worry that the eidolon will never go away,” added Warren. “And when that happens, the eidolon will often move in closer – wrap itself around your arms, or your neck. And they start to take up more and more of your thoughts. Until they control your life completely.”

“Not too long ago, we had a man come in who had been haunted by eidolons for over sixteen years,” said Candice. “They were all he thought about. He’d lost his friends… estranged himself from his family. It was terrible to watch.”

“What happened to the man?” said Seb.

“They’re still haunting him,” said Candice quietly. “We’re getting him help, but – I think it’ll be a long process.”

There was a short silence. Hannah, like her classmates, fixed her eyes on the eidolons, who were still flittering around Candice’s torso.

“So they are dangerous,” said Harry at last. “Sort of. They won’t hurt you by themselves. But they might make you hurt yourself.”

“Yes,” said Candice.

She took a long breath outward, relaxed her neck, and one by one – at a much slower speed than they had descended from – the eidolons rose back up to the ceiling.

“Thanks for coming,” she said. “I hope you’ll discuss what you saw with your parents or guardians tonight. And I hope you’ve learned a thing or two – although, of course, if you haven’t, at least you got a day off from school.”

Hannah found herself running ahead of her classmates as they wound back through the musty, gloomy hallway. She wasn’t sure why, but she did not want to talk about what they had just experienced. As boring as Supernatural Smarts usually was, she felt as though it had been violated, the tedious hours in Warren’s classroom replaced with something dark and disgusting. Which, she argued with herself, made no sense. They were only clouds of glittering black smoke. She had seen that.

“That guy must’ve been kind of pathetic, right?” she muttered to Ella as they got on the bus. “Sixteen years thinking about nothing but eidolons?”

“Dude’s got problems,” Ella agreed.

Hannah ignored Candice’s advice and said nothing to her parents about the field trip that evening. Instead she went straight upstairs and turned on a hip-hop CD that Tom had burned for her. She danced around her room and even tried to sing, ignoring the pained protests of her brothers, who were doing homework downstairs. She kept the volume on high until dinnertime, when her family’s chatter led her thoughts elsewhere.