Easily recoverable, thumb-sized pieces of high-density dry ice are conveniently produced by deposition of carbon dioxide within a test tube submerged in liquid nitrogen. A carbon dioxide-filled balloon sealed over the mouth of the test tube serves as a gas reservoir, and further permits a dramatic demonstration of both the gas-to-solid phase transition and Avogadro's law, with complete deflation of a 2-3 L balloon requiring only about 2-3 minutes. The dry ice "cube" that forms within the test tube is readily removed after slightly warming the tube's outer surface and is of sufficient density to sink in an aqueous indicator solution, permitting its subsequent use for the classic demonstration of carbon dioxide's sublimation and its acidic properties. (Contains 2 figures and 2 notes.)