Leo: But my childhood, for example, I do not remember a thing about it. There is only one thing I still remember.
Leo: The precise moment when I learned how to ride a bike. And this morning. As if by magic I remembered the moment right after...
Alan: When you fell off?
Leo: How the f*ck did you know?
Alan: Well, that happens to everybody. You learn something, you're happy and then. you forget to brake.
I didn't understand What Leo is referring to by say "know" in his reply?, Alan is asking a question: When you fell off?, But Leo replied differently.