Have you ever seen the "concerned children's advertisers" commercial about house hippos? I saw it and for a long time belived house hippos were real. I always wanted to pet one, they're so cute and small. :)
A childhood belief about adverts

others believed things like this
- I thought that hippos were baby elephants— Gerna
- The Whit house down the street was the real "White House". — SG
- I used to believe that when you moved house you swapped house with the person moving into your old house.Anon
- when i was young i believed that when u moved house the house would come with you !!!
just like a turtleAnon - I used to believe that if you opened up an umbrella inside your house that the whole house would fly awayAnon
- When i was small i used to think god lived on top of my house and the devil lived right underneath it.— Missuss D