A pycnometer is a laboratory device used to measure the density or specific gravity of liquids and solids. It typically consists of a glass or metal container with a precise volume, allowing for accurate measurements when filled with a sample. The principle behind its operation is based on the displacement of a liquid, which helps determine the mass of the sample in relation to its volume. To use a pycnometer, the user fills it with a liquid, weighs it, and then replaces the liquid with the sample material. By comparing the weights and volumes, one can calculate the density. This tool is commonly used in fields like chemistry and material science, where understanding the properties of substances is essential.