Which material is most viscous? what are the common uses of viscous materials?

Viscosity is the measure of the resistance of a material (liquid) to gradual deformation, and it is caused by the cohesive forces between the molecules in liquids,. The informal explanation of viscosity corresponds to the thickness of the liquid. The "thicker" the liquid, the bigger the viscosity.

Water is the most viscous substance, with viscosity 1.308 at 10 degrees. The higher the temperature the smaller the viscosity.