PunStuffed teddy bear
Full of food / filled with soft material inside.
Why did the teddy bear say no to dessert? It was already stuffed.
Remember: 'Stuffed' means 'too full to eat' and 'filled with cotton'. A teddy bear is both, so the joke works on two meanings at once.
Short lessonWord playWriters play with words by joining opposites or by giving one word two meanings at once.