I decided to help him find the answer right then and there since we had some mini marshmallows in front of us while we enjoyed our hot cocoa (oh, that wind chill got to me getting Cutie Pie on the bus!) an impromptu experiment was born!
First, I asked him why he thought they floated. He said "because they have air in them." Smart boy.
I grabbed a cup, filled it with water, and we squished a marshmallow and dropped it in (fine motor). It floated, but with only a little surface above water. We dropped in an unsquished one and it floated with almost half of its surface above water.
Sweet Pea declared "The one with the air squished out isn't as good a floater!"
And that's how you do it, answering simple questions when you can and researching the others along with the kiddo if you can!