Dry ice

Dry ice is the popular term for frozen Carbon Dioxide.

Carbon Dioxide freezes at −76.8 °C (−106.2 °F), at normal atmospheric pressure. Once frozen its appearance is similar to water ice that it is called "dry ice". However, unlike water ice, at regular atmospheric pressure, frozen Carbon Dioxide doesn't melt as it heats up. It sublimates -- that that the molecules at the surface will change from solid directly to gas.