I used to think that "lagoon" and "legume" were the same word. I thought a lagoon was so called because it is roughly shaped like a pea pod.
A childhood belief about speaking

others believed things like this
- I used to think that aeroplanes were called "arrowplanes" because they were shaped like an arrow.— Eduardo
- I used to believe that scones were cone-shaped because the word is sCONES. — scone
- I used to believe that the oddly-shaped clouds were made from oddly-shaped sheep— Dickson
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- when i was a kid, I used to think that butterfly shaped pasta could fly and I didn't eat it.— Sabrina in Italy
- i used to believe that stars really were star shaped, i realised at the age of 34 how wrong i was.Anon