This is because the air inside a marshmallow will expand under the heat of a microwave, which is able to penetrate the surface of the candy ... bowl of melted marshmallows, use a microwave safe ...

This is why you want to melt the brown sugar in the microwave in bursts, and check on the sugar and stir it after each burst.

One day while working near the magnetrons that produced microwaves, Spencer noticed a peanut butter candy bar in his pocket had begun to melt - shortly after, the microwave oven was born.