The Inexplicable Giant Floating Baby Head is a mysterious being that appears very randomly. While he/she tends to appear in some of Phineas and Ferb's Big Ideas, both boys are clueless as to where he/she comes from. The baby head is a running gag on the show.
The giant floating baby head first showed up when Phineas and Ferb tried to help Isabella get rid of her hiccups by building a haunted house in the backyard. After Candace went through the haunted house to find her brothers, she complained about how outrageous the house was and asked why the giant floating baby head was even there. The baby head cried after Candace told him/her to be quiet. He/she was also in the haunted house's elevator when Candace was going to use it and was abruptly told to leave the elevator. At the end of the day, Phineas and Ferb relaxed in the tree and thought about what the scariest thing they did was, and Ferb stated that it was the Giant Floating Baby Head. They both suddenly wonder where it came from anyway.
The baby head showed up again as one of the supplies being delivered to the Flynn-Fletcher house when Phineas and Ferb build the monster truck arena in their backyard to help Candace learn how to parallel park although it was not in the arena or in/on the monster truck. He/she reappeared on a handcart behind the boys and Candace when they built the PF 5000 Animatron. The baby head was shown being lifted upwards.
The baby head was also one of Chez Platypus's customers and was allowed by Buford into the restaurant ahead of Candace and Jeremy. He/she was also behind Candace's window as Candace was talking to Stacy about her text message to Jeremy. The baby was being carried by a lever for unknown purposes.
He/she visited Heinz Doofenshmirtz when his foot was broken and somehow managed to sign his cast. He/she was accompanied by Dr. Bloodpudding at the time. He/she is briefly seen in "Livin' in a Funhouse", in the line "The bathtub is your bed and a floating baby head".
He/she was among the various people in Phineas and Ferb building. Before the building's completion, Phineas mentioned putting something next to a giant gorilla head, which is most likely a reference to this. The baby head was also seen during the song "Carpe Diem".
Numerous giant floating baby heads of various sizes appeared in the 4th Dimension during "Brand New Reality", maybe that's where they come from. Months after summer, on Halloween, someone had a baby head costume, he/she was shown a few moments.
Then, the baby head, appeared as an implement for constructing the tower. In Candace's mind, he/she appeared in the song "Going Deep into Your Mind", as the fact, that Candace is afraid of the baby.
Candace was briefly transformed into the Giant Floating Baby Head after she was accidentally transformed into a fly.
The giant baby head came, according to Dan Povenmire, 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."
"Oil on Candace" had a giant baby in it. However, it is not the same as the giant floating baby head as the Baby had a body as well as a different skin color and facial structure.
In the song Technology vs. Nature during the line "when a child cries out in the night, it's rarely for the washing machine" an identical baby with a different hair color is shown.
As of "Misperceived Monotreme" and all its subsequent appearances, the baby head's eyes are more oval shaped rather than the circle shaped eyes seen in previous appearances.
The Giant Floating Baby head appears in the Camp version of the online game Summerizer, behind the mountains when you combine Perry and Agent E.ineas and Ferb are carrying it while Isabella sings Alone.