All Systems No Go
Sometimes Derek wondered if his dad knew him at all. Really, assigning Derek and Casey to work together cleaning? How was that ever going to end well? Anyone else in the house could guess exactly how it would turn out. (Fighting, wrestling, and a bigger mess than when they started. But he had to admit, he had more fun sword-fighting with Casey than with anyone else. She went for the kill.)
He also left Derek in charge while he and Nora went bowling with the Davises. And he let Amy stay. Which was just more proof that George didn't know his son at all - anyone who knew Derek would know he'd see that as a prime opportunity to take advantage of the situation.
Or at least he would have been able to if it wasn't for Casey.
(And maybe George knew that all along which is why he had no problem leaving Derek in charge with a girl present.)
The stupid thing was, Casey didn't even have to try to ruin his date. All she did was come down into the kitchen to have a bowl of ice cream, reading some book that Amy was also in the middle of reading. And from that moment on, it would've been fairer to say that Amy was on a date with Casey, not Derek. After they finished exclaiming over the book in Casey's hand they started talking about another book by the same author, which led to another book, which led to their favourite movies and TV shows and sooner or later, it became clear that Amy and Casey were soul-mates and that Derek had just been standing in their way.
(And what was up with him suddenly dating girls who liked reading and had social consciences? Another ugly side-affect of living with Casey. He needed to go back to his bimbo status quo as soon as possible.)
Even if he had wanted to stand in their way, he couldn't, because he was the babysitter that night and Marti was all high on bubble bath fumes and Lizzie and Edwin had gotten into the coffee ice cream and the less said about that, the better. So, Casey was free to take up all of Amy's attention and he didn't have a free minute to stop her.
By the time Amy's older brother came to pick her up, she was making plans to meet up with Casey the next day and gave her a hug before vaguely waving at him and saying, "I'll maybe see you around, Derek," as she ran out the door.
All he could do was stare incredulously at Casey. How was it possible that she had gotten further with Amy that night than he had?
Casey noticed him staring at her and asked, "What?"
He just shook his head in disbelief and walked away. He could have predicted that Casey would be an obstacle in his dating life (because she loved nothing more than to make sure he was having a rotten day), but he never thought she'd be competition.
(And if he chose to ignore the fact that this could have been avoided if he hadn't asked out a girl who was just like Casey, well, that was his own business.)