When I was little and I heard that someone was "on the wagon," that they were actually sitting on a wagon somewhere. The reason people would "fall off the wagon" is because it either got too crowded, or they got into a fist fight and were pushed. It made a lot more sense when I learned it was a euphamism for quitting drinking.
A childhood belief about speaking

others believed things like this
- I used to think that my grandmother was a pioneer from the covered wagon days.— jasonchic
- That the tooth fairy drove around in her green Volks Wagon Collecting teeth from all the children.— Wanda Trytten
- I believed that if you pushed your belly button, your bum would fall off.— Wendy
- Drinking exactly 3 litres of sunny delight made you turn orange.Anon
- I used to think people had a "potty mouth" if they had been drinking out of the toilet.Anon
- I used to think that the priest at church was JESUS.
He had a beard.... it made sense— Joe B