Matter's three ordinary states -- solid, liquid, gas -- can transform into one another through heat exchange. Heating ice steadily and plotting temperature against time produces a heating curve with two flat plateaus: one at 0 °C (all the incoming heat is melting the ice, not raising its temperature) and one near 100 °C (all the incoming heat is boiling the water into vapour). Melting/solidification describes the solid-liquid transition; vaporisation/condensation describes the liquid-vapour transition; sublimation describes a direct solid-to-vapour transition without an intervening liquid phase (dry ice, iodine). Evaporation (surface-only, at any temperature, with a cooling effect on the liquid left behind) is distinct from boiling (throughout the whole liquid, at one fixed temperature that itself depends on pressure -- rising with increased pressure, falling with decreased pressure, which is why cooking is harder at altitude but faster in a pressure cooker). A pressure-temperature phase diagram, with its vaporisation, fusion and sublimation curves, shows which phase a substance occupies at any pressure/temperature combination, and the single point where all three curves meet -- where solid, liquid and vapour all coexist -- is the triple point. Gas and vapour are distinguished by critical temperature: above it a substance cannot be liquefied by pressure alone (a "gas"); below it, it can ("vapour"). The heat needed to change the state of unit mass without changing temperature is the latent heat, Q=mL, with the latent heat of vaporization always much larger than the latent heat of fusion, because vaporisation must fully separate particles (and do work expanding against the atmosphere) while fusion only loosens the rigid bonds of a solid.