You've put a witness on the stand. On direct examination, the witness testifies beautifully - clearly, coherently, and convincingly. Now you're done - no further questions. Opposing counsel stands, approaches, and proceeds to destroy your witness.
You did not prepare your witness, or you did not properly prepare your witness for cross examination. If your witness was destroyed, it means that he or she provided too much information. Give a good lawyer details, and those details will be probed or twisted or directed to an entirely different conclusion.
Thus, the only four answers your witness should be prepared to truthfully provide to your adversary are:
3. I don't know.
4. I don't remember.
Make it clear to your witness that he or she is not there to help the cross examiner. If a lawyer asks, "How long did [the incident] take?", the inclination of a witness is to be helpful. The witness wants to look precise and answer the question with a definitive time - even though the witness does not know the definitive time it took. The correct - and most truthful - answer is, "I don't know." Any other answer is guessing. And of course, once the witness starts guessing, disaster follows.
Let's say the witness responds, "It took about 5 minutes." The next wily question will be, "Could it have been less [or more] than 5 minutes?" The "could it have been" questions are guess questions, but witnesses don't want to look unreasonable! They try to be rational by saying "yes" (or course it could have been!) or "possibly." Next step if the lawyer wants it to be less time: "So it could have been 4 minutes, right?" Now the witness doesn't know what to say and is probably squirming, and the jury is watching. The lawyer will continue to ratchet down the time as much as possible until it's quite clear that the witness has no idea about the time - thus destroying credibility.
Had the witness begun the exchange honestly, with "I don't know," then every attempt to adjust the time is met with "I don't know" and the cross examiner must give up. To induce your adversary to give up quickly, you jump up at the second question and say "OBJECTION! The witness already stated he doesn't know!" Bested, your adversary will change topics or sit down.
For more details and lessons about preparing your witness to testify, see Chapter 7 of Advocacy Excellence: The Jury Trial.