When my five year old neice was going through the gibberish stage in her language development, a certain phrase, "Ponzi Wonzie," kept recurring. It didn't take long for her to develop the words that she needed to communicate, but Ponzi remained.
By the time she was two she was drawing pictures of Ponzi on the wall; a circle with a face inside it and lines radiating from it. "Ponzi is a boy, but he's a spider today."
After a while we didn't hear from Ponzi any more. Then one day last year we saw a commercial for a new cartoon on the Cartoon Network called "Foster's Home for Imaginary Friends." After seeing that, Marie announced that that is where Ponzi was staying.
Since then Ponzi has been back in our lives full force. Usually he is a boy, but sometimes he is a grown up or even a spider. He has a whole changing ensemble of family and friends, and even his back story changes frequently. (She told me recently that Ponzi was Jewish and that he lived in Africa.) But Ponzi is consistently with us.
Sometimes Ponzi does things that Marie wishes she could do, like drive a car, and sometimes he does things that she does, like take swim lessons. He also does things that get Marie into trouble like making messes.
Occasionally the wind will blow open the front door and she will run to it shouting, "Ponzi! Shut the door when you come in! You'll let the dogs out!" Or if the dogs bark at nothing, she tells Ponzi to quit teasing them.
To me the most fascinating thing about Ponzi is that Marie sometimes uses him as an ice breaker. Whenever she meets some one new, particularly new adults (teachers, etc.) she opens with a Ponzi story. And when they ask who Ponzi is, she says, "Well, he's my imaginary friend."
Now when I run into people who have met Marie and Ponzi, they ask about both of them. In fact, some teenagers who work with my mother are even blaming Ponzi when they get into trouble now.
A childhood belief about imaginary friends

others believed things like this
- I used to believe when dogs bark and cat meow they are talking to each other— frogg gurl
- My dad told me that when he was little, he thought the trees waved back and forth to make the wind blow.— casey
- I used to think that when you get older you could go back and redo things. — cobra
- There was a popular rumor back in grade school that BUBBLE YUM gum was made with spider eggs.— Amber
- I used to tape my mouth shut at night in case a spider would come along and make a web in my belly.Anon
- When I was liitle, I used to believe that if a spider would bite me than I would become Spider-Man!!!— IronManxD