I used to believe that tomatoes came from trees. I discovered that this was false quite late :S
By the way, I seem to remember that I thought the same about watermelons... heheh...
A childhood belief about plants

others believed things like this
- I used to think that girls' anuses were green. I remember thinking that as late as high school.— Jack
- when i was little i thought pigs ate only tomatoes because bacon and ham was red when cooked.Anon
- I used to believe that eating too many tomatoes could make you pregnant.— Mariah
- I used to think that tornadoes were giant tomatoes that would sit on top of your house until everyone died.— Maggie
- I used to think that the title of Barry Manilow's song "Looks Like We Made It" was "Looks Like Tomatoes".— close2nd
- I used to believe that if I ate too many watermelons I would turn into water...— Peter