Requested by phineasis1. Don't know if this is what you're looking for, but the request was somewhat... vague.
"Uh, hey, guys?" Candace unsurely approached her brothers. "As much as I hate to ask you this, can you help me with something?"
"Oh, don't be silly," Phineas responded, misinterpreting the nature of her discontent. "It's no problem at all."
"Heh. Well, my tech class is having this super hard group assignment, but I got stuck as the odd one out. And... I have next to no mechanical skill at all."
Phineas' and Ferb's faces lit up like the Empire State Building on the Fourth of July.
"There are some things we can show you about building."