At 11 years old I changed school to one with separate lessons with specialist teachers. My biology teacher introduced us to the theory of evolution in the first lesson.
It made complete sense to me - if there is a subset of a species with a mutation (adaptation) that makes them ever so slightly more likely to have [more] kids then, over a long time in a large group, those mutations will become increasingly common (because they are genetic) and thus the species evolves. Conversely, any trait that makes it less likely for an animal to reproduce (such as causing them to die quickly) will be strongly selected against.
I came home from school that day and said to my mother, who was a high-school biology teacher herself, "why do people believe that ostriches stick their heads in the sand when they see a predator?"
"Because they do"
"But if they did they would die out very quickly - that's evolution, and it's obviously true whereas 'common knowledge' is sometimes wrong"
"Well it's not wrong in this case"
This incident greatly influenced how I thought of my mother from then on.