I used to believe that larks were very large birds, because, whenever I heard of someone doing something "on a lark", I thought it meant they actually rode on larks to do something, so larks must have been large enough for someone to ride.
A childhood belief about birds

others believed things like this
- I used to believe the large metal structure in Paris, France, I thought it was the "Eye Full Tower"— Janet
- At one time I believed that ZZ Top was an unusually large bustline.— Donna K.
- when i was around 3 or 4 i belived that the sea had a large plug like a bath and it was draind every night — James Cameron
- I used to believe that very large rocks and boulders were actually petrified dinosauer poop.— Maureen
- At nights, when I have trouble sleeping, I used to hear large footsteps - something like the Godzilla.— John
- I used to think the suction of an aeroplane toilet was large enough to suck your kidneys and spleen out.— Big Chrissy S_ _ _ _ _