That's a great question. I don't know the answer, but I can think of one experiment to answer it. Maybe you can think of other experiments to answer it.

My experiments, below, are all based on the fact that, when you put something in a container of water, and push it under water, the level of water in the container will be higher, by an amount equal to the volume of what you pushed under water.

What if you put water in a little measuring cup, and then pushed a marshmallow under the water, and measured how much higher the water level was with the marshmallow under water? If you used your finger to push the marshmallow down, you would want to measure how much higher the water level was with only your finger, and no marshmallow,too.

Then you could mash the marshmallow as flat as possible, and push it back under water again, and see how much higher the water level was with the smashed marshmallow under water.

I don't know the best way to mash the marshmallow. Maybe you could put it between 2 pieces of waxed paper and mash it with your hand or foot or a rolling pin. I don't know how badly the marshmallow would stick to the waxed paper. I think that would be the biggest challenge with the mashed marshmallow - to peel it off whatever it was on when you mashed it.

There are different ways to do the experiment, too. You could push down several marshmallows - maybe 10 - in a bigger measuring cup of water, so it would be easier to see how much the volume changed. You could use a strainer to push down on the marshmallows, because the strainer would probably push a bunch of marshmallows down nicely,without changing the volume too much. When you mashed 10marshmallows, you would have a bigger blob of mashed marshmallows, so you would see a bigger change in volume when you pushed them underwater.

Another way to do the experiments is: You could have a full container of water, and then you could push down the marshmallow[s], and water would overflow out of the container. Then you could measure how much water was in the container before and after you put the marshmallow in it. The difference in these volumes is equal to the volume of the marshmallow[s].

I think it sounds fun to do these experiments.

Keep asking questions!