Lab rabbit
 

One day, a laboratory rabbit escapes from his cage. Outside the compound, he tastes fresh grass for the first time. "This is incredible," he thinks.
He soon finds wild rabbits playing in the fields. "I've just escaped from the lab," he says. "What do you rabbits do?"
"We eat fresh, luscious carrots dug from private gardens," one replies.
So the lab rabbit spends hours eating his fill of succulent carrots. Then he asks, "What else do wild rabbits do?"
"You see that field there?" they reply. "We eat the lettuce." So the rabbit feasts on the fresh, crispy lettuce until his stomach is ready to burst.
"It's fantastic out here!" he cries.
"Are you going to live with us then?" one of the wild rabbits asks.
"I've had a great time, but I can't."
The rabbits stare at him in surprise. "Why? We thought you liked it here."
"I do," the rabbit answers. "But I've got to get back to the lab. I'm dying for a cigarette."

lol

Guess nto many people got this one :D

Heh.

*whooosh*

right over my head.

unless, what i think is the punchline really IS the punchline, and it's just not funny. unlikely.

Sorry, I guess this one was a bit unclear - the main reason for rabbits and rats in labs is to test the effects of things which could be harmful to humans, by force-feeding them high-fat diets, cigs and stuff.