Many witnesses suffer memory lapses in the courtroom. To revive a witness's memory, the attorney can request a recess or ask a leading question. With more serious lapses of memory, the attorney can use a prompting device (such as a document) that the witnesses can use to jar their memories. This is the concept of 'present recollection refreshed.' As a last resort, the attorney can place the document in evidence as a 'past recollection recorded' in lieu of the incompetent witness's recollected testimony. Under the best evidence rule, these documents must be originals; photostatic or mechanical copies are acceptable under the Federal Rules of Evidence. Under the language of perception rule, witnesses are permitted to express opinions only on universal judgments (i.e., regarding alcohol intoxication and auto speed). Expert witnesses' opinions are introduced to assist the jury in understanding the implications of facts. Attorneys must reveal experts' qualifications during direct examination and seek their opinions through hypothetical questions that relate their opinions to the evidence. For rules pertaining to witness cross-examination and impeachment, see NCJ 84340-41.