It started pretty simply. Perry would be at Isabella's right before the time Phineas was usually getting ready for bed. She would call and then either wait for Phineas to pick him or deliver him herself.
Then Perry started sitting on Isabella's lap during the rare quiet moments the boys had. Phineas would usually take the hint and sit down beside her so he could pet him.
Once, Perry even "accidentally" placed himself in front of Isabella while she was walking so that she tripped over him and fell into Phineas, who caught her and smiled.
"I'm onto you, Perry," Ferb said one evening when they were the only ones in the room. "I think they belong together too."
Perry smiled after he left, because…well, he thought so, but he didn't know. Because he didn't remember. Phineas didn't remember. Isabella didn't remember.
But Perry remembered. The kiss…the smile…
And it would happen again. He'd make sure of it.