Liquids have different viscosities, or flow behaviors, depending on the internal molecular structure of the substance, the external forces acting upon it and the ambient conditions, such as temperature and pressure. Internal molecular structure refers to the size and shape of the molecules and the strength of their bonds.
Viscosity is defined as resistance in the flow of a liquid. Liquids with molecules that are large, oval or disc-like and have tighter molecular links have higher viscosities (that is, are thicker and flow more slowly) than those with small and spherical molecules that are not tightly linked. High temperature decreases the viscosity of liquids, because an increase in temperature decreases the force of attraction between molecules.