What you need:
Mini marshmallows
Large clear bowl of water
Cornstarch
Flat surface, such as a cutting board, tabletop or counter
Spoon (optional)
What to do:
Drop a marshmallow into the water. Does it sink or float?
Sprinkle cornstarch over your flat surface.
Put the marshmallow on the cornstarch and squash it. Try rolling it, smashing it down or smashing it between your hands. The cornstarch will keep it from getting too sticky.
Put it in water to see if it floats. (If it sticks to your hands, scrape it off with the spoon.)
The more you squash the marshmallow, the lower it will float in the water. If you squash it small enough, it will sink.
If your marshmallow didn’t sink, grab another one and try again!