Density measurement with a precisely calibrated little bottle, the pycnometer. The analyst weighs the vessel filled with water and then with the sample at a controlled temperature, and the ratio of the two weights gives the liquid’s density. Food laboratories use it to verify syrup concentrations, alcohol strengths, oil identities, and dairy composition, valuing its simplicity and accuracy for routine checks. Careful temperature control and scrupulous cleaning are essential, since tiny errors in volume or residues translate directly into wrong results.