There's this giant disembodied baby human head that shows up in lots of episodes of Phineas and Ferb. It never does anything, it never seems to have any story significance, it's not even funny. It just appears for less than a second, then it's gone.
What's the deal with that? Does the giant head have a back story, whether in universe or out of universe?
Here's a screenshot of the head from "Rollercoaster - The Musical":
Beginning at the wiki, which, while not a reliable source by itself actually provides a source:
The giant baby head came, according to Dan Povenmire, "one board panel drawn by Mike Diederich and one of our directors, Rob Hughes thought it was hilarious and made a whole bit about it. Now it shows up all the time."[1]
This link goes to a page on tampabay.com, which unfortunately leads to a 404 now. However, contrary to what @JohnP said in their answer, I was able to find the link on the Web Archive, at a crawl in 2009.
The relevant portion:
One last question: What is with the giant floating baby head? (It's shown up in several episodes). We like it a lot when Candace says, "What does that even mean?"
Povenmire: That came from one board panel drawn by Mike Diederich and one of our directors, Rob Hughes thought it was hilarious and made a whole bit about it. Now it shows up all the time.
Marsh: We try not to think too much about the floating baby head . . . It makes us laugh, 'nuff said.
In universe, nobody knows exactly where it's from. The wiki is actually very helpful and provides a transcript of the entire episode, for every episode as far as I can tell:
Candace: Phineas! When Mom sees that you've built a haunted house in the backyard with werewolves and vampires and a Giant Floating Baby Head? What's that even about?
[...]
Phineas: That was a great day, Ferb. What did you think the scariest thing was?
Ferb: Definitely the Giant Floating Baby Head.
Phineas: Yeah...
(The ending of the Phineas and Ferb Theme plays.)
Phineas: Yeah, where did that come from?! (Ferb shrugs) Hm.
(The theme continues.)
"One Good Scare Ought To Do It!"
Phineas: We spent a day fighting pharmaceutical zombies,
Would've solved cold-fusion but did fashion instead.
We met Klimpaloon and Meap and the Lake Nose Monster
Phineas, Candace, and Ferb: But we're still baffled by the Giant Floating Baby Head.
"Last Day Of Summer"
My personal theory, though, is that the baby head came from another dimension. We see a whole dimension of baby heads when Phineas, Ferb, Dr. Doofenshmirtz, Perry, Candace, and a whole bunch of evil robots are dimension hopping. At 0:50 in this video, we can see a whole bunch of similar baby heads:
So it would seem that the baby heads came from another dimension and somehow ended up in our normal dimension, or it's somehow magically attracted to Phineas & Ferb's projects... anyway, it appears to come from the baby head dimension.
It appears the Tampa Bay link given in the comments has decayed, (and is not available on archive.org that I can find), however the Phineas and Ferb wiki has the pertinent part cited as part of the background (Credit to Valorum for original links)
The giant baby head came, according to Dan Povenmire, "one board panel drawn by Mike Diederich and one of our directors, Rob Hughes thought it was hilarious and made a whole bit about it. Now it shows up all the time."1
So basically it was a gag that a director thought was hilarious and it stuck. The wiki also has other appearances by said head.
The Floating Baby Head was originally created by Steve Willis and appeared in the nationally distributed comic Bezango Obscuro in 1994.
