Why does Mary go out to the grocers?

After she kills Patrick, Mary desperately needs an alibi. She worries about whether the courts sentence pregnant women to death.... will they execute both her and her unborn child? Not wanting to risk her child's life, Mary immediately begins to prepare her alibi. She puts the lamb in the oven, washes up, tidies her hair and makeup, and then hurries to her usual grocery store. Once there, she tells the grocer that she needs potatoes and peas because Patrick didn't want to eat out and she was "caught . . . without any vegetables in the house." In a moment of truly black comedy, the grocer asks about dessert: "How about afterwards? What are you going to give him for afterwards?" Mary then agrees to purchase a slice of cheesecake.