A/N: As a warning I present Billy as straight in this fic. Still Zack/Billy slash though.

Billy:

Zack leaned into Billy with his most charming smile. Billy crossed his arms, giving him an unimpressed look. Zack smirked and tried to tempt him, "Think of it as an experiment."

"Kissing you would be an experiment?" Billy asked unconvinced.

"Yeah. Jason sucked, how am I supposed to know if I like kissing guys if I end there?" Zack explained.

"Why would you want me to help?" Billy asked blinking.

Zack threw an arm around him, "You're one of my best friends."

"I don't like guys," Billy protested.

"That has nothing to do with it," Zack reassured, "I'm asking you to help me decide if I like guys."

"Because Jason can't kiss?" Bill asked skeptically.

"Jason was awful."

"Why would you think I'd be any better?" Billy frowned.

"How do I know you won't be?" Zack asked leaning in.

Billy scooted away from him. Zack smirked.

"If this is an experiment then what's your dependent variable?" Billy questioned pushing his glasses back up his face.

Zack frowned, thought about it and said slowly, "Me, because I don't change."

Billy nodded, "And what's your independent variable?"

"You and Jason because you're the element of the experiment that I change," Zack smiled.

Billy regarded him, head tilted, for a moment in serious contemplation before he nodded, "Okay."

"Really?" Zack asked, grin widening.

Billy shrugged, "Affirmative."

Zack scooted in closer, "You know, I think I liked it better when you were off balance. You stopped using big words, I actually understood you."

"If you wish to engage in your experiment, then I suggest you do so without preamble as…"

Zack pulled Billy in, kissing him into silence. Their hands shifted restlessly, until Billy's settled around Zack's neck. Zack cupped Billy's face in his hands, moving them slowly from opened mouth kisses to several slow closed mouth kisses. Zack pulled away, eyes darting over his friends face, "Thank you."

Billy opened his eyes and tilted his head blinking.

Zack poked him, "You're supposed to say 'you're welcome.'"

"You're welcome," Billy repeated, before telling him, "I am now quite certain that I shall never be interested in guys."

Zack frowned, "Was I that bad?"

"Absolutely not," Billy reassured quickly, "You were exceptionally good, actually. It's just…this is very difficult to explain…"

"Didn't feel right?" Zack supplied.

Billy nodded, "Exactly."

Zack smirked, "Well, I definitely like kissing guys!"