"The Circus Zebra" joke

The circus was finishing its final performance in the country town when one of its zebras had a stroke. The local veterinarian prescribed a few weeks' rest for the beast, so the circus owner made arrangements to board it at a nearby farm.
The zebra took to the new life immediately and spent the first day meeting all the animals of the barnyard.
He came across a chicken and said, "I'm a zebra, who are you?"
"I'm a chicken," said the chicken.
"What do you do?" asked the zebra.
"I scratch around and lay eggs," said the chicken.
Moving on, the zebra found a cow. He introduced himself saying, "I'm a zebra. Who are you?"
"I'm a cow," said the cow.
"What do you do?" asked the zebra.
"I graze in the field and give milk," said the cow.
The zebra met a bull next. "I'm a zebra," he said. "Who are you?"
"I'm a bull," said the bull.
"And what do you do?" asked the zebra.
"What do I do!" snorted the bull, pawing at the turf with a forefoot. "Why you silly looking ass-take off your pajamas and I'll show you!"