What did you believe as a child that shocked you when you discovered it wasn't true?

When I was very young, my mother played the radio all day to have music and talking in the background. I thought all the people in the commercials and the singers and the announcers were all at the station. I was surprised when I found out that most of them were taped and the music was on records.

I also thought that if I planted a piece of Cheerios cereal that a tree would grow. I had quite an interesting imagination.

What surprised you when you discovered the truth? Were you a firm believer in Santa Claus/Father Christmas well into your teen years? What little childhood beliefs have you forgotten about?