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Video: I will show you an experiment about compression.
First I will try to compress a solid in this syringe. It doesn't work because
the solid can’t be compressed any further.
Next, I will try to compress a liquid (water). It doesn't work either. The particles of the liquid can move freely, but they can't easily be squashed into a smaller volume.
The last syringe has air in it. The particles in air move around very easily and are a long way apart. Because of that, it is much easier to compress air.