Trial lawyers have an old saying: “You never ask a question on cross examination to which you do not know the answer to.”
Want to know why?
Because the answer will surprise you. The answer will shock you. The answer will blow you away. The answer could damage your entire case.
When you cross-examine a witness at trial, you're asking very specific, pointed questions. You are asking leading questions. You do not give the witness an opportunity to explain. You ask questions that call for yes or no answers or true or false answers.
The specific questions you ask are designed to elicit certain answers. It is almost as if you are testifying by the type of questions you are asking.
There are always instances where the attorney will not know an answer to a specific question. In that instance, the attorney would be very well advised to stay away from that subject, unless he simply did not care what the answer was. However, in most cases a good experienced trial attorney will never ask a question to which he first does not know the answer.
As a trial attorney who is questioning a witness on cross examination, you want to make your points and show that your version of the facts are correct and that this witness is simply agreeing with you and your version of events.
The moment you ask a question to which you do not know the answer, you are virtually guaranteed to get an answer that will surprise you and shock you.