The final activity on Wednesday was a lot simpler than the Monday laser show. It was a stunt that every great mechanics lecturer had tried. A bowling ball hung from the ceiling—or, as the case may be, from a bar that held up a basketball hoop—by a cable, free to swing in all directions. The lecturer would hold the bowling ball up an inch from her nose, and drop it, allowing to swing in a wide arc before returning to her face.

As long as the lecturer didn't push the ball, it was a safe demonstration. As long as the lecturer didn't push the ball, it would only have enough energy to reach the same height as it was dropped from, no higher. A good lecturer could hold still. Only the greatest could hold keep their eyes open the entire time.

Michelle did it every week. Unflinching, with a smile.

Elena had tried it once. An involuntary scream had escaped her lips as the ball drew within a few inches of her face. The ball flew towards her, fast enough to invoke fear, just slow enough to leave time for her to ask herself, Did I push the ball? Did I push it? DID I—

And then it was over.

Elena asked Michelle how she did it. "Faith," she'd said.

At the time, Elena gave her a bit of a side-eyed look.

"Don't be afraid of that word," Michelle had told her. "When a nine-pound ball is flying toward your face, you don't have time to do the math. You trust the science, and you trust yourself. You have faith that you're safe."

"I do trust it," Elena had said, "but I still flinch."

Michelle had smiled. "My first summer working this job, I spent an afternoon dropping that ball over and over and over again, watching it swing back towards my face. You know what? It never hit me, not once, whether my eyes were open or closed. At some point, I learned to trust that the physics was going to work every time."

The bowling ball pendulum was always the highlight of Wednesday afternoon, and this week was no exception. The campers gasped, then they applauded and cheered as the bowling ball neared Michelle's face but never touched it.

Standing a few feet behind the circle of campers, Elena grinned as Michelle took a dramatic bow. She could just make out Dr. Baker in her peripheral vision, writing on his clipboard again. His expression was as blank as ever, but Elena couldn't help but believe he was impressed.

This demo wasn't Elena's doing, but he didn't have to know that.

"Okay!" Michelle stepped away from the pendulum, and the bowling ball continued to swing back and forth, its arc shrinking with each cycle. "Who wants to give it a try?"

A stunned silence fell over the crowd, like usual.

"Hey." Michelle crouched forward just a little, making eye contact with each child in turn. "I know some of you have been frustrated by some of the activities so far. Sometimes, you feel like things aren't working right. But you know what?" She grinned with an easy confidence that Elena could never remember having felt as long as she lived. "I've done this a lot of times. I can promise, it always, always works."

Finally, a small hand raised. Everyone cheered and clapped—except Elena, who froze.

Because it was Sam that stepped forward.

Elena's heart beat hard against her will. Michelle's words ran through her head: It never hit me, not once, whether my eyes were open or closed.

The physics would work. Every time. For the kids just as well as it had worked for her. Physics didn't play favorites. It was physics.

I've done this a lot of times.

Elena should have told her more about Sam. This wouldn't be happening if she had just told the whole truth.

I can promise, it always, always works.

Michelle took her place behind Sam and held the bowling ball a few inches in front of Sam's nose.

Cold sweat broke out all over Elena's body. She took a step closer to the circle of kids, then another.

Patrick lead the final countdown— "THREE! TWO! ONE!"—and Michelle released the ball.

Elena watched the ball travel as if in freeze frames. There was no need to panic. This always worked. But then, so did the monochromatic room, and the optical lenses, and the colored beads, and the chemical explosion in the opener show . . .

The bowling ball reached its peak on the other side. It didn't seem higher or lower than usual. But hadn't that been the case for all of the other activities? They seemed to be working fine, until they weren't.

Even if something went awry, there was only a small chance that Sam would even stay in place. Most kids ran from the ball. Elena's gaze shifted from the ball, to Sam's face. His nose was scrunched up, his eyes were closed, his little hands were balled into fists. He wasn't going to move.

He wasn't going to move.

The ball was halfway up to his face.

Elena shoved her way through the group of campers. She heard but didn't register a couple of girls that tumbled to the floor. Her outstretched hands collided with Sam's head as she grabbed onto him.

The force sent him sailing sideways, but she kept her grip and fell with him. The bowling ball just struck her shoulder as she fell, no more than a firm tap.

Sam shouted and clawed at Elena's hands on his head. The cheers had gone silent, given way to gasps and whispers.

Strong hands gripped Elena's upper arms and wrenched her to her feet. Patrick ran over and knelt beside Sam, helping him up. His elbow was skinned white, the skin broken, though not quite bleeding.

Michelle's dangerously soft voice whispered in Elena's ear, "Meet me in the camper" just before she released her iron grip on Elena's arms.

Elena glanced over her shoulder just once as she trudged away from camp, too stupefied and numb to even rub her sore arms. She caught one final glimpse of Sam's face before she no longer dared to look back—his expression bewildered, but his features unharmed.