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.
How it works
A thing appears and the three states are the answers. Three levels change the length of the round and the number of answers offered.
The help asks the question that settles it: does it keep its own shape, or does it take the shape of the jar?
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.