I can't remember if I've used this on the blog before or not, but it's one of my favorite stupid jokes.
Two cows are standing on a hilltop.
First Cow: "Did you hear about this Mad Cow Disease that's going around?"
Second Cow: "Sure did."
First Cow: "Are you worried about it? I am."
Second Cow: "Why should I worry? I'm a penguin."