The classic experiment.
Magnesium ribbon is weighed and burned in a crucible. A lid which is lifted from time to time to allow oxygen in, stops too much smoke escaping. When cool, the crucible is reweighed and an increase in mass is noted.
An interesting variation is to use two crown cork bottle tops held together with nichrome wire instead of a crucible. Air can naturally flow through the loose-fitting sides but very little smoke escapes.