You're A Good Brother

The two of them hadn't talked much this entire time and Roman felt distinctly awkward as the hours stretched on in the motel room with nothing to do. If the silence wasn't broken soon, he was going to lose his mind. 

"So…how did you end up making a time machine?"

Percy blinked at him. "It involved a lot of technical things you wouldn't understand. But the basics were simple enough so I can start there. Time flowed differently in the parallel world. It wasn't a significant difference—only a few days—but it was enough to make me wonder if I could harness those properties to travel through time instead of space.