A clear, sweet mixture of fruit juice cooked with sugar, jelly should be barely firm enough to hold its shape when cut. It is used primarily as a spread for bread and toast or as a filling for cakes and doughnuts. It can be melted over low heat and used as a glaze for fruit, especially on tarts. Natural thickeners in the fruit, called pectin, thicken the mixture. When made at home commercial pectin is often used in the jelly-making process.