Carmen Heredia took the bus with her mother from Tucson to Nogales, Mexico for a dentist appointment. Her two young daughters joined them. On the return trip, they borrowed her Aunt Belia’s car. Her aunt joined the four of them for the trip back into the United States, Heredia behind the wheel. The suspicion of border patrol agents was aroused by the strong smell of detergent in the car. A search turned up nearly 350 lb of marijuana stored in the trunk and wrapped in dryer sheets, apparently to hide the odor of the marijuana. Heredia appeared to be shocked to see the marijuana in the car; she seemed to have had no idea it was there. However, she had had a few opportunities to learn of the presence of the marijuana in the car she would be driving, and she admitted to being suspicious. Her aunt had offered an implausible explanation for the odor, and Heredia did not believe her; her mother seemed very nervous and had a surprisingly large amount of cash, despite being unemployed. But Heredia never looked in the trunk and never queried her mother about the money; she never took the steps she would have needed to take to learn that there was marijuana in the car. Hers is a sympathetic case. Her mother and her aunt were using not just Heredia, but also her children, to cloak their efforts to move drugs into the country. What could be more innocent than a car full of children and the smell of fresh laundry? And yet Heredia was convicted of knowingly transporting marijuana across the border, a conviction that was upheld on appeal. One might have thought that if a statute says that doing something knowingly is a crime, and a particular person did not have the requisite knowledge, then the Principle of Legality, according to which there can be no punishment without a violation of the law, would preclude conviction of the person for the violation of that law. Maybe he could be convicted of violating some other law, but certainly not that one. But, in fact, this is not so.2 The result in Heredia’s case is entirely consistent with the Principle of Legality, given the presence in the law of mens rea substitution principles: principles that tell us that, when a particular mental state, such as knowledge, is an element of a crime, the prosecution can meet its burden by showing something else.