My friend Baz believed that a large truck travelling along a road would stop dead for a tiny fraction of a second each time a fly hit the windscreen (that's a "windshield" for you folks from the US of A). He was aged 33 when he let that one slip!
A childhood belief about physics

others believed things like this
- I believed as a child that the proper name for windscreen wipers was "lig-logs".— Ray
- I wanted to grow up and be a truck. Not a truck driver, a truck. Anon
- At one time I believed that ZZ Top was an unusually large bustline.— Donna K.
- I used to think that if you went to the end of a road marked "Dead End", somebody would try to kill you.— Spooky
- i always thought that a DEAD END sign
as where they put all the road kill.
— pav - I thought when a road sight said Dead End that someone had actually died there.Anon