Sabatoge
Theta looked up from his smoldering experiment, glaring at Koschei. The other boy just smiled innocently and gave a little wave, the de-polarizer still in his hand.
"Kos… that was three days gone up in smoke. Literally."
Koschei shrugged. "Just consider it payback for what you did to me during science class."
"But that was an accident... and you didn't get a bad grade…
"And neither will you. It's just the science fair. C'mon, it was just a little fun."
Theta sighed and looked back at the ashes while Koschei hurried back to his table. Three days of finding the right polarity and containing it in a generator… gone. How on Gallifrey had Koschei found the right frequency of de-polarization to create the chain reaction?
Looking glumly away from the disaster, Theta watched Koschei present his project to the judges. By their expressions, they were impressed.
Suddenly, an evil smile crept onto Theta's face as he pulled out a thin object. It was something he was working on in his free time; a sonic device.
Making sure no one was watching, he aimed it carefully at Koschei's project and pressed the button. A sift wirring noise filled the air, and Koschei's experiment erupted into a light show of sparks. One of the judges screamed from surprise, before they all fled. Koschei stood still mouth open as his work burst into flames. He then slowly looked up at Theta, who smiled innocently and waved, clearly showing the sonic device.
Koschei smiled and mouthed "You win this one."
Credit for this one belongs to both pride1289 and Tsubasa96. Thanks! And, if you have any more ideas, I'd love to hear them!