The Joke:
Why did the teddy bear turn down a slice of cake. He was stuffed.
The Double Meaning of stuff
Literal Meaning (The Bear): Teddy bears are physical toys filled with cotton, polyester, or foam. By definition, their bodies are "stuffed" with material.
Figurative Meaning (The Cake): In human terms, when someone says "I'm stuffed," it means they have eaten so much food that their stomach is full and they cannot eat another bite.
The bear isn't full of food; he's just full of stuffing!