Liu takes Zhao who can't swim to swim. Because Zhao is a minor, Liu takes Zhao out, and he has the obligation of guardianship. In the process of swimming, Zhao failed to fulfill his guardianship obligation and ran to smoke and chat with others, which led to Zhao's death. Its inaction has constituted a crime. The charge should be negligent death.

The unified feature of the crime of omission is that it has the obligation of advance. No obligation does not constitute a crime. This kind of antecedent obligation can be directly stipulated by law, required by position or business, stipulated by contract behavior, or caused by antecedent (dangerous) behavior. This case belongs to the last situation.