Every person is competent to be a witness except as otherwise provided in these rules.
Wis. Stat. § 906.01
The "best evidence rule" requires production of a writing to prove its contents. There is no comparable "better evidence rule" that requires the production of an item rather than testimony about the item. York v. State, 45 Wis. 2d 550, 173 N.W.2d 693 (1970). The trial court may not declare a witness incompetent to testify, except as provided in this section. A witness's credibility is determined by the fact finder. State v. Hanson, 149 Wis. 2d 474, 439 N.W.2d 133 (Ct. App. 1989).