Three other common jokes contrasting mathematicians with other people:<p>A mathematician meets one of his colleagues riding a superb bike. <i>"Where did you get that?"</i>, he asks. The other explains: <i>"Well, there was this beautiful girl riding the bike. When she saw me, she put the bike down, she got naked, and told me to take whatever I wanted."</i> The first one thinks, and says: <i>"You chose wisely: her clothes would probably not have fitted you."</i><p>A mathematician attends a conference about string theory, and seems to be enjoying a demonstration involving thirteen dimensions. The physicist sitting next to him asks: <i>"How do you manage to follow all that?"</i>, and the mathematician retorts: <i>"I can easily visualize it in my head."</i> Impressed, the physicist says: <i>"Wow, you can really visualize it in thirteen dimensions?"</i>, but the mathematician replies: <i>"Of course not, but I just visualize in N dimensions, and then take N=13."</i><p>A physicist comes up with a new equation that seems to fit his theory, and asks his mathematician friend to check it. A while later, the mathematician answers that the equation is false. In the meantime, the physicist used the equation to predict the outcome of a new experiment, and the results were excellent, so he tells his friend to check again. Moments later, the mathematician admits that the equation can be true, but <i>"only in the trivial case of positive real numbers."</i>
An engineer, a physicist and a mathematician are riding the train in Scotland. The engineer looks out the window and spots a black sheep.<p>"Look", he says, "Scottish sheep are black".<p>"No, no", says the physicist, "<i>Some</i> Scottish are black."<p>"No", says the mathematician, "in Scotland there exists at least one sheep, one side of which is black.
A mathematician sees a frog by the side of the road. He stops and the frog says, "Kiss me and i will turn into a beautiful princess and fall in love with you." The mathematician puts the frog in his pocket. The frog asks "Why don't you kiss me?" Replies the mathematician, "I don't have any use for a princess. But a talking frog, that's cool..."
I tend to prefer the dirty math jokes.<p>[NSFW?] The length of my "member" in inches is countably infinite. Just when you think you are all out of inches, you can always find more by inspection.