A volumetric flask is a standard piece of glassware in a chemistry lab. Its main purpose is to be used for creating solutions in with a high accuracy of the concentration.

A volumetric flask containing liquid, the flask is made of clear glass and has a wide base that tapers to a neck. The neck of the flask is long and narrow with straight sides. There is a horizontal red line on the neck of the flask labeled as the calibration mark. Arrows show that the liquid fills the flask exactly to the calibration mark.

Figure 1: Measuring with a volumetric flask.

It often comes with a stopper for the top, which can be used to completely seal of the flask in connection with mixing the content.

As there's only a single calibration mark on the flask, you can't determine the volume of its content with any reasonable accuracy before it's filled to the mark.