In the microwave, an ordinary marshmallow will puff up until it’s enormous!

CAUTION
Kids, please don’t try this without the help of an adult.

1. Put two marshmallows on a paper plate or paper towel.

Tip
DON’T microwave a marshmallow for more than 2 minutes. It will just turn dark brown and make a stinky, sticky mess.

Share & Discuss
How big did your marshmallows get? Measure them and send us a photo of your marshmallow!

2. Put the plate in the microwave. Set the timer for 1 minute (60 seconds) on high.

3. Stand back and watch through the window of the microwave. After about 20 seconds, you’ll see the marshmallows start to puff up. They’ll grow to about four times their original size!

4. When the microwave turns off, take the plate out and put it on the counter.

5. Wait a few seconds, then pull one marshmallow off. Is the marshmallow hollow inside? Is the inside the same color as the outside? When you eat it, is it soft or crunchy?

6. Leave the other marshmallow on the plate and watch it for a minute. When it shrinks back down, you can pull it with your fingers and make it into whatever shape you want. It will stay in that shape and get hard and crunchy. You can eat it, too.

Marshmallows are mostly sugar and water wrapped around a bunch of air bubbles. When you cook marshmallows in your microwave oven, several things happen at once. The microwave makes the water molecules vibrate very quickly—which makes the water heat up. The hot water warms the sugar, which softens a little. The hot water also warms the air bubbles.

When you warm air in a closed container, the gas molecules move around faster and push harder against the walls of the container. As the air in the bubbles warms up, the air molecules bounce around faster and faster and push harder against the bubble walls. Since the sugar walls are warm and soft, the bubbles expand, and the marshmallow puffs up. If it puffs up too much, some air bubbles burst, and the marshmallow deflates like a popped balloon.

When you take the marshmallow out of the microwave and it cools off, the bubbles shrink and the sugar hardens again. When the microwave marshmallow cools, it’s dry and crunchy. We think that’s because some of the water in the marshmallow evaporates when the marshmallow is hot.

If you cook your marshmallow for too long, it turns brown or black inside. That happens when the sugar gets so hot that it starts to burn.

• Expand-a-Face: Dip a toothpick into food coloring and draw a face on your marshmallow before you put it in the microwave. As the marshmallow puffs up, the face will get bigger and bigger.

• In the springtime, it’s fun to expand marshmallow chicks and bunnies instead of regular marshmallows.

This activity is from the Exploratorium publication, The Science Explorer.