Iron rusts at the air and a new substance is formed.  Test what happens with the air during rusting.
What you need: 4 Liquemin bottles, 2 stoppers pierced by drinking straws, red water, two pieces of steel wool.
*  Left: Press steel wool on the bottom of two bottles. * Moisten the steel wool in the bottle marked W, the other one keep dry for control (K).
*  Close the two bottles, push the two tubes only slightly into the stoppers, * Dip them into bottles full of red water.
* Observations middle: The water only climbs up into the bottle with moistend steel wool. 1/5 of it contains water in the end.   * Right: Only the moistened steel wool looks rusty brown.
Explanation:
The iron of the steel wool made a chemical reaction  with oxygen of the air and water. As oxgen disappeared during this reaction the volume of the air decreased inside the bottle.