A hydrometer is a scientific 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 scale on the hydrometer indicates the density of the liquid, which can help determine its concentration or purity. Hydrometers are commonly used in various fields, including brewing, winemaking, and aquarium maintenance. By measuring the specific gravity, users can monitor fermentation processes, assess sugar content, and ensure the proper balance of water conditions for aquatic life.