Since the beginning it has been taught us that matter can be in three very well defined states: solid, liquid and gaseous. When we have a certain amount of water exposed to air, we have two very well defined states, that is, in the bulk of the water we have the liquid state and above the water surface, the gaseous state. But in the limit that links these two phases, an interface exists, with properties totally different from the ones of liquid or gaseous states.

Analyzing the intermolecular interactions that a molecule suffers in the bulk of the liquid phase, we can conclude that the molecule is in equilibrium because them have a null resultant force. A molecule in the surface suffers attractions for the bulk of the liquid, but, in contrast, above the molecule (air) it doesn't suffer any interaction. Thus, in the surface of the water it will not have a null resultant force, doing that the surface water molecules behave in a way that the area is minimum.

That phenomenon is called superficial tension. The superficial tension is defined as a shrinking force that acts in the surface perimeter and that tends to compress it.

In the present experiment, it can be observed that the interactions between water molecules on the surface and others that are below them, make that the liquid overflows. It is worth noting that this phenomenon is very evident for water because it has hydrogen bridges as intermolecular forces. If, for instance, instead of water the experiment was made with benzene, this phenomenon wouldn't be so well notice because the molecular interactions among the molecules of the benzene are much weaker.

In addition, in the experiment it is also proposed the use of an aqueous solution of salt and a plastic cup. In the first case, the addition of salt to the water will increase the superficial tension, because of the ions solvation. That is, the addition of salt will reinforce even more the molecular interactions among the molecules of water. On the other case, the use of two types of materials (glass and plastic) was made to evidence and study the interactions of solid molecules and water. In the plastic cup we will have a larger excess of water, because its walls, by molecular repulses, will impede the formation of drainage channels and the liquid will hold better in the surface. On the other hand, the glass will have higher interaction with the molecules of water having more drainage channels.