Courtney suddenly heard the riddle ringing in her ears once more. "Did you say it's not a chemical reaction?"

"Ja."

"Then maybe that's the answer to the riddle."

"Riddle? What riddle is this?"

"It's just...something I heard," she stammered. "It undergoes no chemical reaction, yet it's reduced by just a fraction. I didn't know what it meant, but now I do. It's radium."

"Or radioactivity," Einstein offered with a wave of his hand as if it was just the simplest thing in the world. "Radium or radioactivity. Mass into energy. Quite simple." He leaned back in his chair and smiled.

"Thanks, Professor Einstein," Courtney blurted out as she pushed up from her chair. "I think it makes sense now. Thank you for the lessons. I'll make sure I tell my friends all about it."

"It was a pleasure instructing someone who shows so much desire to learn," Einstein said.

Before she ran from the room, Courtney paused at the door to take one last look at the brilliant man. It seemed as if he'd already forgotten she'd even been there, for once again he was hunched over his papers, scribbling more numbers and equations in an attempt to solve all the riddles of the universe.

At the far end of the hallway she saw the now-familiar triangle of light. Frantically she looked around for Collin and Morgan. Where were they?

"Collin!" she shouted. "Morgan! We have to go back now. I know the answer. Hurry!" she yelled. "Hurry!"

She stood at the junction of two corridors, looking up and down, listening. She couldn't see her brother and her friend. She couldn't hear them either -- no welcoming shouts, no laughter, no sound of running feet echoing down empty hallways. In the silent darkness, nothing moved.