A hydrometer is a simple instrument used to measure the specific gravity or density of liquids. It typically consists of a sealed glass tube with a weighted bottom that allows it to float upright in the liquid being tested. The level to which the hydrometer sinks indicates the density of the liquid, which can be read on a scale marked on the tube. Hydrometers are commonly used in various industries, including brewing, winemaking, and petroleum, to determine the concentration of solutions or the quality of liquids. They help in assessing the sugar content in grape juice or the alcohol content in beer, providing essential information for production processes.