A man walks into a wine bar with his dog. "Excuse me," says the barman. "No dogs allowed."

"It's OK," the man responds. "This is a super-intelligent, talking dog."

"Oh, yeah?" sneers the barman, "Prove it."

"What grows on trees?" the man asks the animal.

"Bark, bark," replies the dog.

"What do you find on top of a house?"

"Roof, roof," says the mutt.

"What's the opposite of smooth?"

"Rough, rough," growls the hound.

The barman realises he's being made a fool of and throws the man and his dog out.

"Well, I'm terribly sorry about that, Peter," says the dog to his owner outside on the pavement. "Just out of curiosity, which one did I get wrong?"