Second, the scientific knowledge must "assist the trier of fact" in understanding the evidence or determining a fact in issue in the case. The trier of fact is often either a jury or a judge; but other fact finders may exist within the contemplation of the federal rules of evidence. To be helpful to the trier of fact, there must be a "valid scientific connection to the pertinent inquiry as a prerequisite to admissibility." For example, although it is within the purview of scientific knowledge, knowing whether the moon was full on a given night does not typically assist the trier of fact in knowing whether a person was sane when he or she committed a given act.