The brown paper shopping bag is one of the most recurring tropes in all forms of media. This cliche transcends the ages, and is still in use today despite the fact that modern conventions make this particular theme more unlikely with each passing year.
Basically, it reflects the notion that whenever characters return home from a grocery store, they are carrying at least one brown paper shopping bag. Without fail, these bags will contain a loaf of French bread and some leafy green, or the tops of carrots, which will be clearly visible poking out of the top of the bag. Not only is this a standard, but it is practically codified into law. There will ALWAYS be that damned baguette and some indiscernible green leaf sticking up out of the top of the bag. Every time. Without fail.