I used to believe amoebas wait 5 seconds before they start eating something you dropped on the floor.
I'm not sure if amoebas are classified as animals, but I'm putting this in the animals section anyway.
A childhood belief about creepy crawlies

others believed things like this
- I used to think that artichokes were animals because my mother said we were eating artichoke hearts. Anon
- I was told that if you dropped food on the floor, the devil kissed it. — Teresa
- I used to believe when adults grew up only 4' 9" or lower are still classified as kids.— Jorge
- I used to believe that rocks are animals.— Lulzy
- You had to wait an hour after eating ANYTHING, even one M&M, before swimming. Anon
- I used to believe that belly buttons were for putting the salt in while you were eating celery in bed.— David T