Sometimes we denote gas and liquids as fluids. There are important qualitative distinctions between solids and fluids in that a solid can support shear stress while a fluid cannot. What this basically means is that I can make a rod of a solid and hold it from one end while this is certainly not possible for a liquid. Closely related to this facts is the fact that in a solid each atom has a certain average position that it moves around and from which it never strays very far. In a fluid on the other hand atoms move macroscopical distances.
Fluids therefore shape themselves according to their surroundings, ie.
The distinction between a gas and a liquid is quantitative in that the density of a liquid, ie. The higher density indicates that the atoms are closer in a liquid than in a gas. If we denote the linear dimensions of cube which on average contains a single atom by L , then Thus because the density of liquids are three orders of magnitude larger than in a gas the atomic spacings are a typically a factor 10 smaller in liquids.