Set up, in front of the kids, a tall refreshing glass of ice and Coke, or Pepsi if you prefer. Fill to the point of ALMOST overflow. Note the ice cubes are sticking out above the glass edge. Ask kids to predict what will happen as the ice melts. Most will hypothesize that the glass will overflow due to the added volume of the melted ice. Go about your business and come back to this later. They will be pleasantly shocked to note the level has remained constant. This is due to the volume / density ratio of the ice. Why is it floating in the first place?