I should provide a word of caution on this. If you do want to try this out at home, make sure the owner of the microwave has a reasonable sense of adventure about these things. I tried this once in a share house and in one minute of foodie sciencey fun, the owner of the microwave, (unfortunately it wasn’t me), banned me from placing anything else in the microwave.
What was so wrong with microwaving a marshmallow for a minute?
Okay, so it could have been the fact that the marshmallow got bigger in the microwave as it heated up. This is nothing to be worried about. Marshmallows are mostly sugar and water filled with lots of small air bubbles. During the cooking process in the microwave, the water molecules begin to vibrate and this creates heat. The heat causes the marshmallow to soften and the air bubbles also warm.
The gas molecules in the air bubbles begin to move faster and push harder against the marshmallow walls, and because they are now softer, they now allow the air bubbles to expand. It’s a rather delicious demonstration of Charles’s Law which describes the tendency for a gas to expand when heated. The marshmallow continues to expand so long as the bubbles don’t burst. If the bubbles burst, the marshmallow deflates a little.
Once you take the marshmallow out of the microwave the marshmallow collapses as the air bubbles shrink when exposed to the much cooler room temperatures and sugar hardens. And if allowed to cool before eating a warm gooey sweet mess, (This could have been another reason why I was banned but the mess was contained on a plate!), the sugar will harden and the microwaved marshmallow instead of being moist and soft, is now dry and crunchy. This is possibly because water has evaporated during the one minute of heating in the microwave.
Whatever you do, don’t be tempted to leave the marshmallow in the microwave for longer periods as the inside of the marshmallow could brown or blacken as the sugar will begin to caramelise in longer heating periods. Blackened marshmallows don’t taste particularly nice.
[EDIT]: I’ve just been told about Peep Jousting by a Michelle Tourigny in the comments. The link is too good not to share.