Pycnometry is a method used to determine the density of liquids and solids by measuring their mass and volume. The technique involves using a device called a pycnometer, which is a specialized container that can hold a known volume of liquid. By weighing the pycnometer when it is empty and then again when filled with the substance, the density can be calculated using the formula: density equals mass divided by volume. This method is particularly useful in laboratories for analyzing materials in fields such as chemistry and materials science. Pycnometry provides accurate measurements, making it essential for quality control and research applications where precise density values are required.