What happens when a marshmallow is heated?

When a marshmallow is heated, the sugar inside it melts, causing the marshmallow to expand and become soft and gooey. The heat also vaporizes the water content, creating steam that contributes to the expansion. If heated for too long, the sugars can caramelize or even burn, altering the flavor and texture. This process is what makes toasted marshmallows a popular treat, especially when used in s'mores.