Final answer:
The third safety rule in the context of a laboratory safety measures, is to work "one-handed" or keep "one hand in your pocket" when dealing with high-power electric circuits. This practice minimizes the risk of electrical current passing through the chest and heart.
Step-by-step explanation:
The question doesn't specifically mention a context for 'weapon,' but since it's in the context of a laboratory, it appears the student may be confused between general weapon safety and laboratory safety rules. The third safety rule in a general weapons handling context is typically to keep the weapon unloaded until ready to use. However, based on the list provided, for laboratory safety, the third guideline mentioned is related to working with high-power electric circuits: 'When working with high-power electric circuits, it is advised that whenever possible, you work "one-handed" or "keep one hand in your pocket."' This suggestion is sensible because it reduces the chance of electrical current passing through the heart if an accidental shock were to occur, as the current would have a path that does not cross the chest area if only one hand is involved.