I believed that if I stayed in the bathtub too long, I would rust - thanks to my sister.
My mom also convinced me that she could make 'Magic Water' that could cure any sickness. I believed her until I walked in on her putting food coloring in ice water.
I thought that if I started the bath water, because it was so loud, it would wake the sharks down the drain and they would come up and eat me.
Oh, and, when we lived in the apartments, all the kids were absolutely sure that the apple-things in the park were poisonous and would kill you if you even licked it. Which I did. And I'm still alive, as far as I know..
I thought that everything on television stopped once you turned it off, like it was on pause, and would just continue when you turned it back on. It didn't. D:
I was dead sure that my mom was magic because she used to tell me that she could see the future when we got to stoplights. She would watch the stoplights on either side of us and when they turned red and ours was about to turn green, she would snap and it would turn green. I eventually figured out how she did it on my own and she started laughing. xD
Every time one of my parents left for work, I thought they were never going to come back and bawled for at least an hour. But when my sister left for school, I didn't care if she was coming back. xD
And, of course, I believed in Santa, the Tooth Fairy, and the Easter Bunny. Well, sort of. I think I always knew that the Easter Bunny didn't exist. Or I was just too scared to even consider a huge rabbit hopping around and laying colorful eggs in my backyard at night.