Right:  Three quarter fill a vial with water.
* Add a small piece of black substance  from an old 9-Volt battery (MnO₂ = manganese dioxide).
* Add as much 30% hydrogen peroxide (H₂O₂)  solution as possible.
* Close the vial with a stopper punctured by a drinking straw.
* The vial full of liquid is placed bottom up on a second 5-ml vial:   left side on the left photo.
Observations right side on the left photo: A gas is collected in the upper vial while the liquid is pressed into the lower one. The vial with gas is tightly closed. Keep it for the following experiment.