Why Frances Has Trouble in School

Here's an exchange between Frances and myself:

Me: "What does a reindeer do when it's being chased by a polar bear?" (In my head I'm thinking about the great giggle we'll have about the answer: He runs for deer life. hehehe)

Frances: "Polar bears live at the bottom of the world. Reindeer live with Santa Claus at the top."

Me: "Okay, yeah. But it's a joke."

Frances: "And polar bears don't chase reindeer. They eat fish."

Me: "Let's just pretend."

Frances: "Well, the joke can't be funny because it doesn't work like that."

Me: "Fine." I had to laugh at my joke all by myself.