If you consider the molecules of solids, liquids, and gases, you can see how they are in fact very different.
the molecules of a solid (such as ice) are stuck together in a rigid form.
The molecules of a liquid (water) are a little less stuck together, but they do have their boundaries.
The molecules of a gas (steam) have no boundaries, as steam does not have a definite volume. This means the molecules are free to roam around all over the place.