To thicken is to make a liquid denser and more viscid so that it flows less easily — one of the most fundamental operations in cooking. Cooks thicken by several mechanisms: starch gelatinisation, as when flour, cornflour, or arrowroot swell in a sauce; reduction, boiling off water to concentrate the remaining solids; emulsification, as in mayonnaise; and gelling with gelatin, pectin, or agar. Each gives a different texture: a roux-thickened sauce is opaque and hearty, a reduction is glossy and intense, a cornflour slurry is clear and slick. The skill lies in judging the final consistency, since thickened liquids continue to set as they cool, and in avoiding lumps by dispersing the thickener properly.