I thought opium was an element.
One time, I was playing in the living room while my mother was watching something and a character on the TV said, "Oh! I forgot to close the fridge door!", and so Mum turned to me and said something along the lines of, "Don't you ever forget to close the fridge door."
Then later, I saw an episode of "Spongebob Squarepants" (which is this wacky cartoon about a talking sea sponge) where Spongebob gets sick from leaving his fridge open.
Both of those made me think that every time you left the fridge door open, something random, but always very bad, would happen.
When I was about eleven, I knew about the periodic table and I thought the expression "exposed to the elements" meant being exposed to the actual elements, like on the periodic table, and I thought, "surely that's everywhere?".
I used to think that in order to top up your mobile phone you had to open the back of the phone and put actual money in it. And I thought that when you phoned people the phone would actually eat your money.
My cousin told me that "radioactive" meant "evil".
I used to believe that hitting the Break key on the keyboard would cause the computer to be ruined.
When I was little, I noticed that when the other person hung up, there would be a series of beeping sounds followed by a whine. I'd freak out because I thought that meant the phone would explode if I didn't put it back onto the receiver pronto.
When I was little I thought there really were aliens on mars.
when i was i child i used to believe that everyday there was a new sun in the sky.
When I was a child, I used to believe that the force of gravity was an old woman named "Marialonga" sucking down children with her hands.