Imagine a story in which your protagonist is the victim of a poisoning. And the mode in which the poison was delivered was marshmallows! 

Does she die in the first scene? If so, how do you tell her story? The reasons that someone hated her enough to want her dead would be revealed as the story spins.

Or, perhaps your protagonist is the poisoner. Why would she choose marshmallows and not, say, a bottle of wine? Does the intended victim have a passion for the simple marshmallow?

How does the poisoner make sure the intended victim is the one who consumes the poison, and not some innocent child?

Your task is to tell the story in such a way that readers will be enthralled.

Narrative is important in setting the scene. Dialogue is needed to establish the tense relationship between the victim and the poisoner.

Rely on sensory details. Readers need to see, imagine the taste, smell and touch of the marshmallows. And to watch the suffering of the victim. Is it a quick or slow death?

Have fun with this one.