his demonstration introduces the students to the idea that large objects do not necessarily have a higher density than small objects
Density depends on how much mass is packed into a given volume (i.e., if you had the same volume of two things, the one with more mass would have a higher density).
Just because something has more mass (or is bigger) does not mean it is has a higher density. A quick way of describing density is to describe an object as heavy or light for its size.
Pumice stone, unlike regular rock, does not sink in water because it has a low density. Pumice stone is igneous rock formed when lava cools quickly above ground (lava froth). You can clearly see where little pockets of air have formed.
Ironwood is a common name for a large number of species of wood that are very hard. Our “local” species, the American Hop Hornbeam (Ostrya virginiana), is found in central and southern Ontario. This very hard wood is resistant to compression and is valued for making tool handles and fence posts. An ironwood branch is very dense and sinks in water.