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A01 Air can be compressed and expanded

Take an "empty" plastic bottle and close it tightly. Try to crumble the bottle by standing on it. Observation: The pressure of your body is not sufficient to crumble the bottle as it is full of air.
By pumping air into the inner tube of your bicycle you know that air can be compressed. Left:  This closed bottle was photographed 20 m under water by a scuba diver. It is extremely compressed!
Right: These two 3-ml syringes contained 2 ml of air, when their openings were closed by welding.
Observation: The air locked in the syringe can be compressed and expanded by pushing and pulling the piston. Conclusion: If you measure the volume of air you also have to know its pressure.