"Mad Cow Disease" joke

Two cows are grazing in a pasture enjoying their grass. At around noon they decide to lay under a tree, chew their cud, and talk for awhile.

The first cow says to the other, "Have you heard about that awful new Mad Cow Disease?"

The second cow, chewing her cud says, "Yea, I heard about."

The first cow says, "Well, I'm really worried! I heard that a lot of the cows in the pasture down the road have caught it!"

The second cow says, "Yea I heard."

The first cow says, "Well you don't seem to worried about it!"

The second cow says, "I'm Not worried."

Irate now, the first cow says, "how can you just lay there, and not worry about such a horrible Disease!?"

The second cow says, "Easy, because I can't catch it"

The first cow says, "And Just What Makes Think That!!!"

The second says, "Because you stupid Cow, Can't you see I'm a Duck!!!"