Sugar 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. 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.