You Snuck Your Way Right Into My Heart - Love Handel (from Phineas and Ferb)

And some extras.

I should’ve known
From how I felt
When we were together
And even more when we were apart
You tiptoed in
And you got under my skin
You snuck your way right into my heart

I put up barriers
To shield my emotions
A wall that you could never break apart
But like a ninja of love
Repelling down from above
You snuck your way right into my heart
Oh yeah