The Fairly OddParents - 1x04b - Tiny Timmy!

In order to study about the human body for the school project, Cosmo and Wanda give Timmy a Shrinky Suit so that he can explore the microscopic world, but after Cosmo is accidentally swallowed by Vicky, Timmy must enter her body to get him and Wanda out. While inside, he learns many interesting things about the human body and Vicky, including that the brain is a room with little men at control screens, and a black hole occupies the spot of Vicky's heart. After messing around in Vicky's brain and turning her into a sweet and kind Princess, her antibodies suspect he is an intruder and react naturally: by setting out to destroy Timmy!