Canadian Border Services This is part way down the page:
In-transit travel through Canada
If you are an American resident, you can transport goods through Canada to the United States. To simplify the clearance process, carry three copies of the list of goods you are transporting. The list should include the description and value of the goods, as well as the serial numbers (if applicable). You should pack consumable goods such as alcohol, tobacco and food in containers that the border services officers can close and seal when you arrive.
Basically Canada Customs will put a seal on your container, when you enter Canada, US customs will inspect that seal to make sure it's intact when you return to the US.