Until I was about 9, I believed farts were actually bubbles...rather unpleasant brown bubbles, to be specific. Farts sounded different because some involved just a few big bubbles bursting loudly, while others were the result of a large number of small bubbles popping in rapid succession. These bubbles also explained why one's underwear sometimes ended up stained at the end of the day. All quite internally consistent, when you think about it.
A childhood belief about farting

others believed things like this
- Because i could not see them, i thought my farts came out as green bubblesAnon
- I used to think when you farted, bubbles came out.— Nick
- I used to believe that eyes where just like bubbles and if you where to touch them they would pop.— Adam
- I used to believe that if I swallowed gum I would bubbles when I 'trumped'.Anon
- If i farted in the tub the bubbles would be different shapes.Anon
- I used to belive that, if you swallowed your chewing gum, you'd fart bubbles...— Heidi, Pittsburgh, PA