Isabella came home one afternoon. Earlier today, Phineas invited her for a ride on a flying carpet he and Ferb invented. She had a few hours until dinner, so she decided to check her E-Mail. There was a message from Ferb called 'Mermaid Theory'.
"Mermaid Theory?" Isabella thought. "What's that?" She opened the E-Mail.
This is why Phineas appears to be clueless when it comes to your crush.
The 10-year-old girl clicked on the link, which opened a page in the How I Met Your Mother Wiki. She read the article.
"Sailors stuck at sea would get desperate for female companionship. It got so bad that eventually the manatees out in the water started to look like beautiful women: Mermaids. You see, every woman, no matter how initially repugnant, has a Mermaid Clock, the time it takes for you to realize you want to bone her. Sure, today you see [your secretary] Iris as a manatee, but she ain't going to stay that way."
It took Isabella a while to realize what that meant: even the plain and simple-looking girls will eventually become attractive in a boy's perspective.
In other words, she had hope.
A few weeks later, Phineas and Ferb were out on the road with their family. They just finished with their truck stop diner, but it was accidentally removed from the RV by a low bridge.
"Hey Ferb," Phineas said. "What was that E-Mail you sent Isabella a while ago?"
"A Wiki article about The Mermaid Theory," Ferb replied. "I learned about it from a sitcom Candace watches."
"Mermaid Theory?" Phineas asked. "Are you planning on searching for them as one of our projects?"
"The Mermaid Theory has nothing to do with actual mermaids," Ferb explained. "According to comedian Neil Patrick Harris, no matter how unattractive a girl is, a boy would eventually see her as the most beautiful girl he ever saw. One day, you might see a girl from across the street as a manatee, but you might see her as a mermaid as time passes."
Phineas thought about it. Somehow, he imagined Isabella wearing a manatee suit. "That's crazy," he said.
"Just remember that theory," Ferb said. "You'll thank me later."
Days passed since Ferb told Isabella and Phineas about The Mermaid Theory. It was already August 6th, but no one could remember what happened yesterday.
"I'm serious, Ferb!" Phineas exclaimed. "The last thing I remember is doing a musical number about Perry, then it was already time for dinner, and for some unknown reason, my mouth feels weird."
At that moment, Isabella arrived in the backyard. "Watcha doin'?" she asked.
Phineas was about to respond, but he couldn't. Yesterday, he kept on seeing Isabella wearing a manatee suit, but now he was seeing her wearing a mermaid costume and sitting on a rock. A blush quickly appeared on his face and he felt happy for absolutely no reason. He was so distracted, he didn't pay attention to what he randomly told the 10-year-old girl.
"You're the most beautiful girl I ever saw, I love you, and I want you to be my girlfriend."
Ferb snorted a laugh. "Thank you, Neil Patrick Harris," he thought.