My grandfather has the heart of a lion... and a lifetime ban from the zoo.
Lion Heart
Explanation
Explain Bear
Alright listen up, this is a simple one, even for you. The joke is that the guy's grandfather is supposed to be brave because he has "the heart of a lion". But, plot twist, he's banned from the zoo, implying he stole the lion's heart. Which is also why you should stay away from the zoo, dumbski.