Gases spread out to fill their containers. Students can't normally see this. But iodine sublimes to form a purple gas so students could see it filling a covered beaker as it is heated. This Powerpoint presentation has animations showing this and other demonstrations of how liquids and gases behave differently. (A free worksheet is available on my website to assist you to actually do these demonstrations, or to use along with the animations at http://www.evanschemistrycorner.com/WS/PhasesWS/WS6-2-1_Comparing_Solids_Liquids_and_Gases.pdf.) In addition, the presentation defines each of the physical phases, and explains why they behave differently based on kinetic molecular theory.