A simple and easy way to explain weight and balance is to demonstrate it. Using a scale you can demonstrate that the three masses (solid, liquid, gas) all have weight.
For example: On your scale when the levels are equal, explain the term balance. Clarify that both sides of the scale are equal in weight and that's why both sides of the scale are equal in height. Next, you can place different solids on both sides or perhaps the same object but in different amounts. Explain that whatever side falls down farther, is heavier. This can also be done using a liquid. Now gas will be a little different. Blow up two balloons of equal size and tape them to both sides of the scale. Explain that gas (air) has weight just like solids and liquids. Next, tear a small hole in one of the balloons. That side should start to rise, while the other side begins to fall.
I hope this answers your question