A joke about ecology.

When the Chinese New Year was approaching, a group of animals were shipwrecked and drifted on the sea in a small boat. The food was running out, and they decided to throw away the extra companions through games. The rule is to tell jokes in turn. If someone's joke can't make all the animals laugh, it's out of luck!

Cattle first. The cow's joke was so wonderful that almost all the animals laughed, but the pig didn't. So the cow was thrown into the sea. Then it's the sheep's turn. Sheep's jokes suck. No one can laugh. But the pig smiled

"What are you laughing at?" The animals asked in surprise.

"Sorry, I remembered the joke of Niu." The pig answered slowly.