Between molecules always appear bond forces (Van der Waals or hydrogen bonds). These binding forces can be "illustrated" by the volume contraction in mixing fluids. By mixing different liquids together, the volumes do not add, there occurs a volume contraction by binding forces between the molecules. This experiment shows this volume contraction by the mixing of water and methylated spirits. The students find out that a mixture of 25 ml water and 25 ml of ethanol does not lead to a total volume of 50 ml. The volume difference can be used as a "proof" for the binding forces between water and ethanol.