A few years ago there was a conversation I had with a six or seven year old. We're at a lake and the boat is coming in to take him for 'tubing'.
While we're waiting, I tell him to watch out for the sharks.
The kid turns around with a confused look and says: "I thought sharks were a salt water fish; why would they be in this lake?".
I had a mental freeze for a second or two from the shock of this reply. As I recovered my composure, I looked straight in his eyes and said without flinching: "......exactly......your tube's here....".
Gringo