I heard one about a lawyer who was questioning a doctor. It went something like this:

Lawyer: "Did you take the man's pulse before you pronounced him dead?"
Doctor: "No, I did not."
"Did you check for breathing or a heartbeat?"
"No, I did not."
"So, it was possible that the patient was still alive!"
"No."
"How can you be so sure?"
"Well, his brain was sitting in a jar on my desk, but for all I know, he could be out practicing law somewhere."