When does a cub become a Boy Scout?
When he eats his first brownie.
Cub Scout to Boy Scout
Explanation
Explain Bear
Alright listen up, dummy. This joke is funny because a "Cub Scout" becomes a "Boy Scout" when he eats a brownie. "Cub" sounds like "cubby" and all of us bears like brownies. It's funny cause bears like brownies and humans are dumb.