http://www.slate.com/articles/arts/culturebox/2015/06/phineas_and_ferb_finale_interview_with_show_creators_dan_povenmire_and_swampy.html

One night in a Pasadena restaurant in the mid-’90s, the animator Dan Povenmire—then working as a writer on the Nickelodeon series Rocko’s Modern Life—doodled a simple cartoon on the butcher paper covering his table: a triangle-headed boy with a pair of round bubble eyes and a mouth agape in nervous alarm. It’s an expression that’s hard to imagine on the wedge-shaped face of Phineas Flynn, the unflappably cheerful young inventor that triangle-headed kid would become—but mouth shape aside, the character is pretty much fully present in that first sketch. As Povenmire would recall when Phineas and Ferb finally had its premiere on the Disney Channel nearly 16 years later, “I tore that piece off and told my wife, ‘This is the show I'm going to sell.’ ”