A hydrometer is a simple instrument used to measure the density or specific gravity 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 liquid's density, which can be useful in various applications, such as brewing, winemaking, and measuring battery electrolyte levels. Hydrometers come in different types, including alcohol hydrometers for measuring the alcohol content in beverages and battery hydrometers for checking the charge level in lead-acid batteries. By providing a quick and easy way to assess liquid properties, hydrometers are valuable tools in both scientific and industrial settings.