The three classical phases of matter on Earth are solid, liquid or gas. Phase transitions occur with sufficient change in pressure and/or temperature. For example, water (liquid) transitions to ice (solid) with a drop in temperature. Gelatin powder, such as Kraft Foods’ Jell-O, is a solid. Empty a packet of Jell-O into a mixing bowl and add boiling water. Stir until dissolved and then chill. Now the material in the bowl is neither solid nor liquid nor gas; it’s a hydrogel.