Phineas and Ferb Season 2 Episode 48
The Lizard Whisperer
Phineas and Ferb find a chameleon in the backyard, who grows to the size of a dinosaur, thanks to Doofenshmirtz's Gigant-Inator, and takes a walk through downtown. Meanwhile, Jeremy teaches a song to Doofenshmirtz, who plans to use it to summon aliens to do his bidding.