What special kind of glass is used in film and TV special effects to shatter without hurting the actors?

Sugar glass (also called candy glass, edible glass, and breakaway glass) is a brittle transparent form of sugar that looks like glass. It can be formed into a sheet that looks like flat glass or an object, such as a bottle or drinking glass. It is used to simulate glass in movies, photographs and plays. It is much less likely to cause injuries than real glass, and it easily breaks convincingly, making it an excellent choice for stunts.

Sugar glass is made by dissolving sugar in water and heating it to at least the "hard crack" stage (approx. 150 °C / 300 °F) in the candy making process. Glucose or corn syrup is used to prevent the sugar from recrystallizing, by getting in the way of the sugar molecules forming crystals. Cream of tartar also helps by turning the sugar into glucose and fructose.

Because sugar glass is hygroscopic, it must be used soon after preparation, or it will soften and lose its brittle quality.