When I was young, I figured out why yawns were "contagious." When someone yawned, they sucked in a lot of oxygen, leaving less for everyone else in the vicinity, so people nearby had to yawn too, to get more into their lungs. I still sorta believed it into my 20s.
A childhood belief about chemistry

others believed things like this
- My sister told me once that if I sucked too much helium out of a balloon, my lungs would explode.Anon
- When I was young I used to believe for some reason that being asthmatic meant you were born with no lungs.Anon
- i used to believe that if you burped and yawned at the same time your eyes would pop out— kelso
- I used to believe that if I stood at the top of the stairs, closed my eyes and yawned, I flew down them— Sam
- My older brother told me that everytime you sneezed your lungs would bleed. I believed this until I was 21!— Gem
- I used to think that oxymoron had something to do with oxygen...Anon