The low-pressure phase diagrams of water (H2O, left) and carbon dioxide
                    (CO2, right).  The different areas of these diagrams represent conditions under which
                    these substances are gaseous, liquid, or solid.  At points 'A' and 'D', they are solid; at1
                    'C', 'E', and 'F, gaseous; at 'B', liquid.  The thick solid lines represent transitions1
                    between phases.  Along these lines, two phases can exist together.  At the triple1
                    point, where the lines intersect, the solid, liquid, and gas can all co-exist.  For H2O, the
                    triple point falls at a temperature of 0.0099 oC and a pressure of about 0.006 times the
                    pressure of Earth's atmosphere at sea level. For CO2, it falls at -56.6 oC and 5.1 atmospheres.
                    The heavy dashed lines represent sequences of heating (horizontal) or squeezing (vertical):

                    1. Start with the ices at 'A'. Keeping the pressure constant, warm them until they1
                    reach their solid-liquid transitions. At this point the ices melt. Further heating initially1
                    results in warmer liquids ('B'). If you warm the liquids enough, they reach their1
                    liquid-gas transitions and evaporate. Further heating results in warmer gases ('C').

                    2. Start with the ices at 'D'. Raising their temperature does not cause them to run into1
                    their solid-liquid transitions, but rather into their solid-gas transitions. The ices1
                    evaporate directly into the gas phase in a process called sublimation. Further1
                    heating warms the resulting gases ('E' and 'F').

                    3. Start with gases at 'E'. Holding their temperature constant, squeeze them until they1
                    reach their solid-gas transitions. At this point the gases condense directly into solids1
                    ('A'). For CO2 and most other materials, more pressure will not produce any additional
                    changes ('G' on right diagram). For H2O, however, additional pressure pushes the
                    solid to the solid-liquid transition, where it melts ('G' on left diagram).

                    4. Start with gases at 'F'. Squeezing them pushes them to their solid-liquid transitions,1
                    where they condense directly into liquids ('B'). For CO2 and most other materials,
                    further squeezing pushes the material to the solid-liquid transition, where it solidifies1
                    ('H'). For H2O, squeezing the liquid doesn't do anything (until reaching extremely
                    high pressures off the top of the diagram).