Played in the episode
No More Bunny Buisness, when Phineas and Ferb test out their new X-Ray glasses after being ripped off of from another pair they ordered.
Here are the lyrics -
They say that beauty's just skin deep
But I got a brand new perspective
Your skeleton is symmetrical
and your vascular system's effective
You can't keep secrets from me 'cause I can always see them
with my X-ray eyes
I got a brand new way of lookin' at life
an all together diffrent visual spectrum
sometimes photons behave like a wave
but they're particles when you reflect 'em
But that don't bother me 'cause either way I see it
with my X-ray eyes
with my X-ray eyes
with my X-ray eyes