What it teaches
The three states, and how to tell them apart: a solid keeps its shape, a liquid takes the shape of what holds it, and a gas fills whatever it is in.
Gas is the hard one, because most gases cannot be seen. Air, steam and the bubbles in a fizzy drink are the three a child has actually met, and they are the ones here.
What to look for
Sand and flour are worth talking about. They pour like a liquid and each grain is a solid, and that is exactly the kind of case that makes the rule stick once it is resolved.