Pitch or tar is a viscoelastic polymer. It appears to be solid at room temperature and can be shattered with a hammer, it is actually fluid and will flow, but extremely slowly. A piece of pitch put in a glass jar and allowed to drip at the University of Queensland in 1930 to measured the flow rate has produced eight droplets so far (2011). It is calculated that the viscosity of this pitch is 230 billion times that of water.

Pitch is not the only seemingly solid substance that will flow. Many materials appear to be solid, but they have a non crystalline amorphous structure (tar is different. It is made up of long chains of hydrocarbons, which become entangled and exhibit extensive cross linking, prohibiting flow for the most part). These materials (which include chocolate mousse, shaving cream, mayonnaise, metallic glasses, granular materials and mud) are 'amorphous solids'. They are resistant like solids but, like liquids, lack a crystalline structure. These materials also flow under the force of gravity.

Glass windows hundreds of years old have been 'observed' to bulge at the bottom because glass has flowed down from the top. In fact this is incorrect. Glass is not a fluid and does not flow, in spite of the fact that it has a non crystalline amorphous structure. The bulge at the bottom of many old windows is due to the way the glass was manufactured.

Amorphous solids are different from solids in that there is no well defined phase transition between being a solid and liquid. In particular, there is no latent heat of fusion, indicating that the nature of the bonds formed does not change. Though glass also does not have latent heat of fusion, there does appear to be a phase transition from liquid to solid showing that glass is ina solid unlike many other amorphous solids.