The typical experiment to measure the oxygen content in the atmosphere uses the rusting of steel wool inside a closed volume of air. Two key aspects of this experiment that make possible a successful measurement of the content of oxygen in the air are the use of a closed atmosphere and the use of a chemical reaction that involves the oxidation of a solid element into a solid oxide. The oxygen for the chemical reaction is provided by the closed volume of air, whose volume changes are easy to measure. In previous versions of the steel wool experiment, several days were needed to complete it; however, the experiment can be completed in about 30 min by cleaning the steel wool using acetone and then washing it in acetic acid or vinegar. A new simple closed-volume experiment to measure the oxygen content in the air is described that can be completed in a few minutes. In this experiment, steel wool is placed inside a glass tube that is connected with two normal glass syringes. The tube is then heated by a flame and the air in the syringes is forced, by moving the syringes plungers, to pass through the heated steel wool. This experiment is an excellent classroom illustration of the 21% of O2 in the atmosphere, and it can be used as a complement to the steel wool - acetic acid experiment.