My sister and I used to think that we were the inventors of nachos and cheese. One day we saw our other (older) sister eating nachos and cheese and we got so mad because we didn't give her permission to eat that food.
I also thought that stuffed animals came alive in your sleep, those weird grey seashells on the beach were whale poop, if you kept digging in the sand at the beach you'd hit like cement or something, if you swung on the swing hard enough you could circle all the way around the top pole (I can't tell you how many times I tried to do that!), I was fascinated by where food went when you ate it and where poop and pee came from (I saw no connection between the two!), if you think hard enough you could move a pencil, when I was little my brother told me that our cabbage plants ate people (I never got near the garden again), I also thought that the world used to be black and white (like in those old pictures), I didn't believe in aging, The drain at the bottom of the pool was evil!, the deep end of the pool went on and on and if you held your breath long enough you could go to some magical place or something, I can't think of any others...