Children often think that the 'heavier' an item is, the more likely it is to sink. This is a key misconception that needs to be addressed from the beginning. In the module, the key sinks while the rubber duck floats - but is the mass of the key greater than the mass of the rubber duck? Not necessarily! You could have the most massive rubber duck in the world and it would still not sink in water.
Making the connection that density is proportional to mass, when the volume is kept constant. This is addressed in the module where the child 'weighs' different substances which have the same dimensions. They find that the denser objects have the greater mass for a given volume.
Understanding that the density of both the liquid and the solid are important in determining whether the solid will float or sink in that liquid. The module addresses this in a simple way by allowing the children to 'experiment' with different liquids as well as different objects. E.g. the egg floats in salty water but sinks in normal water.
The idea of an average density e.g. in a ship or the human body is challenging for this level. It can be approached using images and examples.
For instance, perhaps the child could consider items with air in them as being able to float, even if the density of the material itself is greater than water. E.g. the ship made of steel has lots of air in it - it is not solid steel.