I have my second graders use their index finger as a testing finger for solids, liquids, and gases. They ask 2 questions while using their fingers: Can you put your finger thru it?
Can you pour it?
If you get 2 no's, then it's a solid.
2 yes's, then it's a liquid.
1 no, 1 yes, then it's a gas.
Another thing we do is act out how matter behaves. They each become a molecule of matter. First we bunch up together as tightly as we can so that we can't move, then we're a solid. Then we take a step backward and we can move more freely, then we're a liquid. After that we move way far apart where we can float all over, then we're a gas! They always understand it!