Complex sentence: A mule is someone who, knowingly or unknowingly smuggles something onto an airplane or across a national border, sometimes aware that they are smuggling, but not knowing what is in what he or she smuggles.
Split into simpler sentences: A mule is someone who, knowingly or unknowingly smuggles something onto an airplane or across a national border. Sometimes the mule is aware that they are smuggling, but do not know what is in what he or she smuggles.