I used to believe that tranquilizer darts make people fall asleep within 10 seconds. I also didn't realize it's possible to accidentally make a killer dart instead of a tranquilizer dart if you overcalculate the drug dosage.
A childhood belief about chemistry

others believed things like this
- I used to believe that "tranquilizer" was short for "tranquilizer dart".Anon
- when i was a kid, i used to think that if i didn't fall asleep, morning wouldn't comeAnon
- I used to believe that if you point at somebody you make holes in the air and the fairys fall over— Bob #1
- i used to think that biting my nails would make my fingers fall off! ;)— Cassandra
- for some time I used to think that at a drug store people really sold drugs.— NEO_ryu
- I thought Pink's "You make me sick" was "you make me sick, you make me look like a candlestick" LOL— Rice