A childhood belief about cars

When I was pretty little, I liked to stick my head out of the car window as we were driving. My dad told me that if I stuck my head out there as we were driving to close to something my head would of course be chopped off. Now, I never sat behind my dad, because he always had his seat pushed back so far it was really uncomfortable. Therefore it never occurred to me that he could be talking about other cars. There was never anything on the outside side of the car but trees, and it didn't seem like a tree branch could really cut your head off, it just wasn't sharp enough. I concluded that there must be a certain kind of tree that had razor-sharp branches specifiacally designed to defend it from little kids hanging out of car windows.