Because space is three dimensional (it has length, width and height) we use cubic units to measure volume. In the metric system the main units of volume are:
cubic millimetres or mm³
cubic centimetres or cm³
cubic metres or m³
a litre is also a metric measure of volume, which is the space taken by a 10 cm by 10 cm by 10 cm container.
A cubic millimetre is incredibly tiny, each side being just a millimetre in length. Each side of a cubic centimetre is 1 cm long. A cubic metre can be made out of canes or one metre rulers and it surprising how children like to sit inside the final cube!