You shouldn't lie about having a medical condition.
Marge offers Melissa, who is perfectly healthy, a burnt cupcake that Marge cooked in class.
Melissa doesn't want to hurt Marge's feelings.
Melissa eats the cupcake after dipping it in some sauce, and gives Marge some baking tips.
Melissa tells Marge that she has diabetes;therefore, eating the cupcake would make her sick.
Marge listens to Melissa becoming a better cook.
Marge is upset that Melissa lied to her when she discovers the truth.