In cooking, thickening is the process of increasing the viscosity of a liquid either by reduction, or by the addition of a thickening agent , typically containing starch.
Desserts are often thickened with sago, tapioca, gelatin or a gelatine substitute such as agar. Soups, sauces and stews are more often thickened with a starchy product like cornstarch, arrowroot or wheat flour, or a fat and flour mixture such as roux or beurre manié. More rarely, savoury dishes may be thickened with blood.