The Lean Mean Brain Machine

Phineas and Ferb build a brain switching machine to see what it's like in other peoples' shoes, who end up switching brains with Perry the Platypus and RHF and become Perr (Perry in Ferb's body), Ferbeas (Ferb in Phineas's body), Phin-fu (Phineas in RHF's body (known as Rufu in Japan)), and Ruffy the Platypus (RHF in Perry's body).